Physiology · Unit 1: The internal environment and its control

Endocrine Axes and How to Test Them

How do you find where a hormone axis has failed when many of its hormones move in pulses and daily rhythms? This lesson traces four anterior-pituitary axes, labels their long, short and ultra-short feedback loops with the evidence for each, and shows how paired trophic and target hormones, suppression and stimulation tests, and the choice of assay each narrow the answer. Afterwards you can read a hormone pair through its feedback loop, say what a challenge test can and cannot separate, and explain why the same number means different things on different assays.

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Film transcript: Read the Pair, Not One Number

One blood sample gives you one number. What can it tell you? A hormone level is one point on a moving curve. Hormones are released in pulses, then cleared. Growth hormone left plasma with an average half-life of twenty-seven minutes. That was one small study's mean, after a large intravenous dose in five people. A sample drawn just before a pulse can read many times lower than one drawn on its peak. So read two hormones together. Take the cortisol axis. CRH, helped by vasopressin, drives ACTH from the pituitary. ACTH drives the adrenal cortex to make cortisol. Call ACTH the trophic hormone and cortisol the target hormone. Cortisol then feeds back on CRH and ACTH.

That is long-loop feedback, with strong evidence at both levels. Now break the chain at the gland. In this made-up example, adults on a normal sleep-wake schedule have blood drawn at 08:00. Person A has cortisol of 3 µg per dL and ACTH of 300 pg per mL. Low cortisol should make an intact pituitary drive hard, and it does. The adrenal does not answer: that is the primary pattern. Reference intervals differ by laboratory and assay. Person B has the same low cortisol, but ACTH is only 5 pg per mL. The drive itself is missing. That points to the pituitary, a secondary disorder, or to the hypothalamus, a tertiary one. The pair cannot say which, because either lesion lowers ACTH.

It can separate gland disease from central disease. Now change one input and watch the output. Person B receives synthetic ACTH. Thirty minutes later, cortisol is 16 µg per dL. Whether that is adequate depends on the assay, because criteria for newer, more specific assays sit below the historical one. An adequate response shows only that the adrenal can answer ACTH. After a recent loss of central drive, the adrenal cortex may not yet have atrophied. So a normal result does not exclude a recent pituitary or hypothalamic defect. The thyroid reads the same way. High TSH with low free T4 is the classical primary pattern. Low free T4 with low or inappropriately normal TSH points to central disease.

A hormone level is a point on a moving curve, and a criterion belongs to its assay. Read hormones as pairs, and know which assay made the number.

In 1968, seven of eight young adults sampled through the night showed a large surge of growth hormone as they entered deep sleep, and when sleep was delayed by about three hours in four of the eight, the surge was delayed with it in the three who showed one.

A hormone level is one point on a moving curve. This lesson labels the feedback loops of four anterior-pituitary axes and asks what one sample, a pair of samples, a challenge test or the assay itself can tell you about where an axis has failed.

Five axes, one wiring plan: a releasing signal, a pituitary hormone, a target

The endocrine system introduced the hypothalamus, portal vessels and anterior pituitary, and hypothalamus and autonomic control tells how the portal link was shown. The plan is a relay, like a chain of managers: a hypothalamic signal in portal blood, a pituitary hormone, a target tissue.

  • Growth hormone (GH). GHRH (growth hormone-releasing hormone) stimulates somatotrophs; somatostatin, in the same portal blood, inhibits them. GH makes the liver secrete IGF-1 (insulin-like growth factor 1), and GH also acts directly through its own receptors, for example promoting breakdown of stored fat in adipose tissue, so IGF-1 does not carry every GH effect. GH and prolactin receptors signal through JAK2 and STAT5 (JAK-STAT).
  • Prolactin. This axis runs backwards. Hypothalamic dopamine tonically inhibits prolactin release through lactotroph D2 receptors. Several factors, TRH among them, can stimulate prolactin in experiments, but no releasing hormone dominates. Milk production belongs to pregnancy, birth and lactation.
  • ACTH. CRH (corticotropin-releasing hormone), helped by vasopressin from the same hypothalamic neurons, drives ACTH, and ACTH drives the adrenal cortex to make cortisol (the adrenal glands).
  • TSH. TRH drives TSH, and TSH stimulation is required for the thyroid to make enough T4 and T3 (thyroid hormone).
  • LH and FSH. Pulsatile GnRH drives both (animal reproduction).

For the MCAT headings: releasing hormones, pituitary hormones and dopamine are peptides or amines acting mostly on G protein-coupled receptors (GH and prolactin are exceptions: their receptors use JAK2) and second messengers such as cyclic AMP and calcium (GPCRs and G proteins, cAMP and PKA). Cortisol and thyroid hormone travel mostly on carrier proteins and act on nuclear receptors. Target specificity is receptor expression: every cell sees ACTH, and only cells with ACTH receptors respond.

Feedback comes in long, short and ultra-short loops

A feedback arrow is named by its origin. Long-loop feedback runs from the target-gland hormone back to the hypothalamus and pituitary. Short-loop feedback runs from a pituitary hormone back to the hypothalamus. Ultra-short feedback is a hormone acting back on the cells that make it, classically a hypothalamic releasing hormone acting on its own neurons. The term is sometimes also used for a pituitary hormone acting on its own cells.

ArrowTypeEvidence
Cortisol on CRH and ACTHLongStrong, at both levels
T4 and T3 on TRH and TSHLongStrong, at both levels
IGF-1 on GHLongStrong, human infusion
Prolactin on dopamine neuronsShortCausal, knockout mice
GH on GHRH neuronsShortCausal, cell-specific knockout mice
TSH or ACTH on hypothalamusShortLess established
Prolactin on lactotrophsUltra-shortSuggested by cell culture, of proliferation not secretion

The GH long loop was tested in people. Bermann, Barkan and colleagues (1994) gave six normal fed men a saline infusion followed, in the same admission, by a 48-hour infusion of recombinant IGF-I, sampling GH every 10 minutes for 24 hours in each period. Overall GH fell by 85 plus or minus 3 percent and pulse amplitude by 77 plus or minus 4 percent (mean plus or minus SE). The authors favoured increased hypothalamic somatostatin, an inference from hormone responses; nobody sampled portal blood.

The modern experiment tests a short loop: prolactin is thought to stimulate the dopamine neurons that inhibit it. Schuff, Low and colleagues (2002) compared mice lacking D2 receptors, mice lacking prolactin receptors and compound mutants, used D2 agonist and antagonist challenges, and cultured pituitary cells. If prolactin acted only by driving dopamine, the prolactin receptor knockout should look like the D2 knockout. Females lacking either receptor developed hyperprolactinemia and large prolactinomas, but prolactin receptor knockouts had more hyperprolactinemia than age-matched D2 knockouts and had lost much of their dopamine tone. Prolactin also slowed proliferation of normal female lactotrophs in culture. So the loop runs through the hypothalamus and also acts at the pituitary independently of dopamine, which fits an ultra-short action on lactotroph proliferation, shown in cultured cells. Global, lifelong deletions cannot assign each phenotype to one cell population, and the results are mainly in female mice.

For GH, deleting GH receptors only in GHRH-expressing mouse cells raised pulsatile GH secretion and pulse amplitude, yet acute GH or IGF-1 did not change GHRH-neuron firing in recordings (Gusmao and colleagues, 2025). Short-loop feedback need not mean fast electrical inhibition. TSH and ACTH short loops rest on older, thinner evidence, and their role in humans is not settled.

Hormones keep time: pulses, daily rhythms and sleep

The classic experiment asked what times the night-time GH rise. Takahashi, Kipnis and Daughaday (1968) studied eight young adults over 38 nights, drawing repeated blood samples through an indwelling catheter while EEG and eye-movement recordings staged sleep. If a clock or the overnight fall in glucose set the rise, delaying sleep should leave the peak in place. In seven of the eight, a major GH peak appeared at the onset of deep (slow-wave) sleep and lasted 1.5 to 3.5 hours. Delaying sleep onset by about three hours in four subjects delayed the peak in the three who had one, three subjects woken for two to three hours showed another peak on falling asleep again, and the peak did not correlate with concurrent changes in glucose, insulin or cortisol. That ties the surge to sleep onset in these young adults and argues against a simple glucose or insulin trigger. It does not show the hypothalamic mechanism (more GHRH or less somatostatin), that the pattern holds at every age, or that glucose never regulates GH. Sleep staging is in circadian rhythms and sleep.

The HPA axis keeps a daily rhythm with pulses on top. In humans under normal conditions on a usual sleep-wake schedule, it peaks between 06:00 and 10:00 and reaches its nadir between 22:00 and 02:00, with ACTH and cortisol pulses on that curve; the rodent pulse experiments are in emotion and stress.

Pulses matter for measurement because of clearance. Parker, Utiger and Daughaday (1962) injected 5 or 10 mg of human GH intravenously into five people and found that plasma levels fell with an average half-life of 27 minutes (individual values 23 to 38). That is one small study’s mean after a large dose, not a constant; later deconvolution of naturally secreted GH in healthy adults gave shorter values, about 15 minutes. Either way the scale is the same: a pulse decays to a quarter or less within an hour, and a sample drawn just before a pulse can read many-fold lower than one drawn on its peak. A random GH value is weak evidence, and in adults a single random GH or IGF-1 value is not considered enough, in most patients, to establish GH deficiency. IGF-1 serves as an integrated readout because it circulates mostly bound to binding proteins and is considered to have a much longer half-life than GH, so it averages many pulses, though it also depends on nutrition, liver function and age.

One sample, many answers — a pulsatile hormone⚙ original · interactive
Sleep onset (illustrative times; moves the large pulse)
Single sample reads (relative units)
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24-hour mean (relative units)
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Lowest and highest over the 24 h
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Same sample drawn 10 min later
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The reading, step by step
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In this sketch, the status appears here.

In this sketch: the 27-minute average half-life is the only measured number (people, average after large intravenous doses of GH in a 1962 study of five people; individual values 23 to 38 minutes, and not a constant). Everything else is made up for the sketch: a daytime pulse is 1 relative unit and the sleep-onset pulse is 3; pulses come every interval from 07:00 until sleep onset and none follow it; release is instantaneous (real secretion is not instantaneous; in the 1968 study of eight young adults, seven had a major peak at the onset of deep sleep that lasted 1.5 to 3.5 hours); a later onset also adds daytime pulses; there is no baseline secretion and nothing carries over from the day before.

Keep this: with a 27-minute average half-life, a single sample mostly reflects the last hour, so one reading can depend on when it was drawn as much as on how much was secreted, and a mean from frequent sampling over 24 hours is steadier.

Paired levels locate the lesion; dynamic tests ask about reserve

A primary disorder starts in the target gland, a secondary one in the pituitary, a tertiary one in the hypothalamus. Treat the trophic hormone as the pituitary’s report on its target: a low target hormone should make an intact pituitary drive hard.

Worked problem: reading three pairs. In this made-up example, three adults on a normal sleep-wake schedule, taking no glucocorticoid and with normal cortisol-binding globulin, have blood drawn at 08:00. The numbers are illustrative, the ACTH values are invented, and reference intervals differ by laboratory and assay.

PersonCortisol (µg per dL)ACTH (pg per mL)
A3300
B35
C1430

Step 1, convert. Cortisol’s molar mass is about 362.5 g per mol, so 1 µg per dL = 10 µg per L = (10 x 10^-6 g per L) / (362.5 g per mol) = 2.76 x 10^-8 mol per L, or 27.6 nmol per L.

cortisol (nmol per L) = cortisol (µg per dL) x 27.6

A and B: 3 x 27.6 = 82.8 nmol per L. C: 14 x 27.6 = 386.4 nmol per L.

Step 2, read each pair. A: low cortisol, high ACTH. The pituitary senses low cortisol and drives hard; the adrenal does not answer: primary. B: low cortisol, low ACTH. The drive is missing: secondary or tertiary, and the pair cannot say which, because either lesion lowers ACTH. C: morning cortisol in the expected direction with mid-range ACTH; no failure shown at rest, though a value of 14 µg per dL does not by itself prove normal reserve if adrenal insufficiency is suspected.

Step 3, stimulate. B receives a dose of synthetic ACTH (cosyntropin, ACTH 1-24), and cortisol 30 minutes later is 16 µg per dL, or 16 x 27.6 = 441.6 nmol per L. Whether that is an adequate response depends on the assay: criteria derived in 2021 for newer, more specific cortisol assays sit below the historical criterion from older immunoassays.

Step 4, interpret. An adequate response shows only that the adrenal can answer ACTH. After a recent loss of central drive the adrenal cortex may not yet have atrophied and can respond normally, so a normal result does not exclude a recent pituitary or hypothalamic defect. Answers: A primary; B secondary or tertiary, unresolved; C no failure shown; B’s 441.6 nmol per L takes its label from the assay.

The thyroid reads the same way: high TSH with low free T4 is the classical primary pattern, while low free T4 with low or inappropriately normal TSH points to central disease.

Dynamic tests change one input and watch the output. Suppression tests probe autonomy. A night-time dose of dexamethasone, a synthetic glucocorticoid, normally lowers ACTH and next-morning cortisol; failure to suppress suggests secretion escaping feedback but does not locate the source, and faster dexamethasone clearance or more binding globulin can confound it. Oral glucose normally suppresses GH, and failure to suppress helps when GH excess is in question. Stimulation tests probe reserve: synthetic ACTH tests the adrenal, and GH stimulation tests (insulin-induced hypoglycemia, glucagon, or macimorelin, a ghrelin-receptor agonist) test whether the GH axis can release GH; several act through the hypothalamus rather than on somatotrophs directly. A test that stimulates a gland directly cannot say whether a weak response reflects the gland itself or a long-standing loss of drive from above, and a normal response does not exclude a recent upstream defect.

Test the wiring: a compressed stalk. A non-secreting mass presses on the pituitary stalk, and portal delivery falls. Hormones driven by releasing hormones tend to fall (ACTH, TSH, GH, LH and FSH), in combinations that vary from patient to patient, and with them cortisol, free T4, IGF-1 and sex steroids; TSH may be low or inappropriately normal rather than high. Prolactin rises, usually modestly, because the signal it loses is inhibitory dopamine. A drug that blocks D2 receptors, as many antipsychotics do, also raises prolactin, with no lesion at all, and a D2 agonist lowers prolactin whether the cause is a stalk lesion or a prolactinoma. So a drug response does not locate the problem. What points to the stalk is a modest prolactin rise together with low levels of other pituitary hormones.

A number is only as good as its assay

Competitive radioimmunoassay is explained in detecting a signal. In a competitive assay, labelled and sample hormone compete for a limited antibody, so bound signal falls as hormone rises. In a two-site (sandwich) assay, a capture antibody and a signal antibody bind two sites on one molecule, so signal rises with hormone. Sandwich assays suit large peptides such as TSH; small molecules such as free T4 are usually measured competitively.

Both measure immunoreactivity, what an antibody recognizes, not bioactivity, what a receptor responds to. Macroprolactin, a large prolactin-IgG complex, is detected by immunoassays but has low biological activity, so a high reported prolactin can mean little at the receptor. Antibodies also cross-react with related molecules, a problem for steroids. LC-MS/MS (liquid chromatography-tandem mass spectrometry) separates the hormone from its relatives and identifies it by mass and fragments, so it reports a defined chemical species. Better specificity changes the numbers, so a criterion from an older assay can mislabel a newer result.

Then there is binding. Roughly 99.97 percent of circulating T4 is protein-bound in commonly quoted figures, and published values vary a little with method. Free hormone is the unbound fraction, considered the part available to tissues (binding and dose-response). Total T4 moves with binding protein: more thyroxine-binding globulin, as in pregnancy or with oral estrogen, raises total T4 while free T4 can stay near its usual range. Physical separation by equilibrium dialysis followed by isotope-dilution LC-MS/MS is a reference approach for free thyroid hormones; routine free-hormone immunoassays are method-dependent estimates.

Design problem

In a 2020 study, ten healthy adults took a biotin supplement daily for a week, at a lower and later a higher amount, and their thyroid tests were run on three platforms. Two hours after the last dose of the first regimen, one platform reported lower TSH and higher free T4 than baseline, a pattern that looks hyperthyroid, while the tested assays on another showed no significant change. A healthy volunteer taking biotin shows that pattern. Design tests that separate a real change in thyroid function from an artefact of measurement.

One way to do it

Start with the mechanism. The standard explanation is that some immunoassays capture the antibody complex with biotin and streptavidin. Excess sample biotin occupies the streptavidin, less complex is captured, and signal falls. In a sandwich assay (TSH) less signal reads as less hormone; in a competitive assay (free T4) it reads as more. Both errors point toward “hyperthyroid”.

Rerun the same sample on a platform without a biotin capture step. A biological change should appear on both; an artefact only on the susceptible one.

Repeat after stopping biotin. In this study the interference lasted about one to two days after the higher regimen on the susceptible systems, so a repeat after that should return to baseline if the change was analytical. This small study does not set a universal washout.

Look for physiological agreement: a real hyperthyroid state usually also moves heart rate and symptoms, though older adults may show few, and T3 measured on a platform without a biotin step, because T3 assays on the susceptible platform can be biased too.

The study did not test larger doses, impaired kidney function, other analytes or current versions of every assay.

How we measure it

Frequent blood sampling with sleep staging

Place an indwelling venous catheter so blood can be drawn every 10 to 30 minutes with little disturbance to the subject, and record EEG and eye movements to stage sleep. The paired records separate short secretory pulses from the daily rhythm and link release to a state such as sleep onset.

Competitive and two-site (sandwich) immunoassays

In a competitive assay, sample hormone and a labelled hormone compete for a limited antibody, so bound label falls as hormone rises. In a sandwich assay, a capture antibody and a signal antibody bind two sites on one hormone molecule, so signal rises with hormone. Both report what an antibody recognizes, which is not always what a receptor responds to.

Liquid chromatography-tandem mass spectrometry (LC-MS/MS) with equilibrium dialysis

Chromatography separates the hormone from related molecules, and two stages of mass spectrometry identify it by its mass and fragments and quantify it. Dialysing serum across a membrane first, until free hormone equilibrates, lets the method measure the free fraction directly rather than estimate it.

Gene knockout mice, global and cell-specific

Delete a receptor gene in every cell, or only in one cell type with a Cre recombinase driven by that cell's promoter, and ask which feedback arrow disappears. Lifelong deletion can let other parts of the axis adapt, so a phenotype is evidence about a loop, not a direct reading of its strength in adults.

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Human growth hormone bound to two receptor extracellular domains — the complex that starts signaling
Human growth hormone bound to two receptor extracellular domains — the complex that starts signaling ● LIVE · RCSB PDB 3HHR
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One hormone molecule, two receptor molecules. Find the two different surfaces of growth hormone that each grip one receptor: the receptors are already paired before GH arrives, and the hormone changes how the pair is arranged, which activates the JAK2 kinases inside the cell.

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William H. Daughaday

William H. Daughaday 1918–2013

Senior author of the 1962 study that measured how fast injected growth hormone leaves human plasma and of the 1968 study, with Takahashi and Kipnis, that tied the night-time growth hormone surge to the onset of deep sleep rather than to clock time.

The Endocrine Society's memorial records that William Daughaday joined Carl and Gerty Cori's laboratory as a research fellow in 1949. Two of his group's experiments sit at the centre of this lesson. In 1962, with Parker and Utiger, he followed human growth hormone injected into a vein and found that it left plasma with an average half-life of 27 minutes after large intravenous doses in five people. In 1968, with Takahashi and Kipnis, his group applied the same logic: if the hormone clears so fast, frequent sampling can show when it is released. Over 38 nights in eight young adults, with sleep staged by EEG, seven showed a large surge at the onset of deep sleep, and the surge followed sleep onset in the three subjects whose sleep was delayed and who had a surge. With W. D. Salmon he had earlier described a serum "sulfation factor" that acted on cartilage and depended on pituitary growth hormone, the line of work that led to IGF-1. The work made growth hormone a hormone you could time as well as measure.

✓ verbatim Takahashi, Kipnis and Daughaday, Growth hormone secretion during sleep, Journal of Clinical Investigation 47:2079-2090 (1968); Rotwein, In memoriam: William H. Daughaday, MD (1918-2013), Journal of Clinical Endocrinology and Metabolism 98:4217-4218 (2013).
Ariel L. Barkan

Ariel L. Barkan

Senior author of the 1994 study in which a 48-hour infusion of recombinant IGF-I suppressed pulsatile growth hormone in healthy men, the clearest human test of long-loop feedback in this axis, and of a 2019 review of how common endocrine assays fail.

Ariel Barkan, an endocrinologist (a University of Michigan faculty member from 1983), studied how the growth hormone axis is regulated in people, where you cannot sample the portal blood that carries hypothalamic signals. His group's approach was to change one input and watch the pulses. In the 1994 study with first author Bermann, six normal fed men received a saline infusion followed, in the same admission, by a 48-hour infusion of recombinant IGF-I, with blood drawn every 10 minutes for 24 hours in each period. Overall growth hormone fell by 85 plus or minus 3 percent and the amplitude of spontaneous pulses by 77 plus or minus 4 percent (mean plus or minus SE). The authors judged the hormonal pattern most compatible with increased hypothalamic somatostatin, an inference, since somatostatin was not measured in portal blood. Barkan was later senior author, with Haddad and Giacherio, of a review of how common endocrine assays go wrong: competitive and sandwich designs, interfering antibodies, macro-complexes, cross-reacting steroids and biotin. The two papers frame the two halves of this lesson: feedback you can test in a person by changing one input, and numbers you can trust once you know how they were made.

✓ verbatim Bermann, Jaffe, Tsai, DeMott-Friberg and Barkan, Negative feedback regulation of pulsatile growth hormone secretion by insulin-like growth factor I, Journal of Clinical Investigation 94:138-145 (1994); Haddad, Giacherio and Barkan, Clinical Diabetes and Endocrinology 5:12 (2019).
Malcolm J. Low

Malcolm J. Low

Senior author of the 2002 mouse study that compared dopamine D2 receptor knockouts with prolactin receptor knockouts and found that prolactin's feedback runs both through hypothalamic dopamine and directly at the pituitary.

Malcolm Low's laboratory studied how hypothalamic neurons and pituitary cells regulate one another, using mice from which a single gene had been removed. Prolactin is the odd hormone of the anterior pituitary: its main hypothalamic signal is dopamine, which inhibits it, and prolactin was thought to feed back by stimulating those dopamine neurons. That model makes a testable prediction. If prolactin acts only through dopamine, a mouse without prolactin receptors should look like a mouse without dopamine D2 receptors. In the 2002 study with first author Schuff, and with Binart and Kelly among the co-authors, females lacking either receptor developed hyperprolactinemia and large prolactinomas. The prolactin receptor knockouts had more hyperprolactinemia than age-matched D2 knockouts, and drug challenges showed they had lost much of their dopamine tone. Prolactin also slowed the proliferation of lactotrophs from normal female mice in culture. The prediction failed in an informative way: the loop runs through the hypothalamus and also acts on the pituitary itself. The study gave a genetic test of a feedback loop that is often drawn as a single arrow.

✓ verbatim Schuff, Hentges, Kelly, Binart, Kelly, Iuvone, Asa and Low, Lack of prolactin receptor signaling in mice results in lactotroph proliferation and prolactinomas by dopamine-dependent and -independent mechanisms, Journal of Clinical Investigation 110:973-981 (2002).
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In adult mammals, which hypothalamic signal dominates the control of prolactin secretion by lactotrophs?
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Every structure, slide, simulation, and clip on this page is listed below with the source it came from. None of the 3D structures is drawn by hand or by AI: each one loads by accession ID from RCSB PDB or PubChem. This panel is generated from the lesson's own data file, so what it lists is what the page loads.

Molecular structures live data

  • Human growth hormone bound to two receptor extracellular domains — the complex that starts signaling — RCSB PDB 3HHR · coordinates CC0 · Crystal structure of human growth hormone with two copies of the extracellular domain of its receptor (de Vos, Ultsch and Kossiakoff, Science 1992).

Simulations

  • One sample, many answers · Original interactive — Biochemistrypedia (Deppmann Lab).

Scientist profiles & quotes

  • William H. Daughaday — Takahashi, Kipnis and Daughaday, Growth hormone secretion during sleep, Journal of Clinical Investigation 47:2079-2090 (1968); Rotwein, In memoriam: William H. Daughaday, MD (1918-2013), Journal of Clinical Endocrinology and Metabolism 98:4217-4218 (2013).
  • Ariel L. Barkan — Bermann, Jaffe, Tsai, DeMott-Friberg and Barkan, Negative feedback regulation of pulsatile growth hormone secretion by insulin-like growth factor I, Journal of Clinical Investigation 94:138-145 (1994); Haddad, Giacherio and Barkan, Clinical Diabetes and Endocrinology 5:12 (2019).
  • Malcolm J. Low — Schuff, Hentges, Kelly, Binart, Kelly, Iuvone, Asa and Low, Lack of prolactin receptor signaling in mice results in lactotroph proliferation and prolactinomas by dopamine-dependent and -independent mechanisms, Journal of Clinical Investigation 110:973-981 (2002).

Quotes are verbatim from the cited source. Profile text is original to this site and was checked against the sources cited.

Animated explainer

Read the Pair, Not One Number · narration: an AI narrator voice (Kokoro, af_heart) reading narration written from this lesson's text · animation: AI-generated in code (Manim) by Claude, a schematic with no drawn molecular structures, plots computed from their equations · captions from the narration script.

Images & figures

This lesson carries no raster figures. Every structure is rendered live from RCSB PDB or PubChem, and every plot is drawn from its equation in the browser.

integrity Machines check the mechanics. The build confirms that every local file exists, that every question's answer key points at a real choice, and that any AI-generated scientific figure names the person who checked it; a weekly job re-resolves every accession ID against its database. Questions are original; MCAT-style items are written for this site and are not real AAMC questions. Banned sources (the Tymoczko textbook, publisher decks, copyrighted figures) are not used.

Like I'm five

Your brain sends tiny messages to a small gland under it, and that gland sends messages to other glands around your body. When those glands have made enough, their message travels back and tells the brain to ease off, like a thermostat. Some messages come in quick puffs, and a big one comes when you fall deeply asleep. So one drop of blood can catch a puff or miss it.
High school

The hypothalamus releases signals into small blood vessels that run straight to the anterior pituitary, which then releases hormones that act on the liver, adrenal glands and thyroid. The hormones those glands make travel back and turn the system down, a negative feedback loop. Prolactin is the exception: the brain mostly holds it down with dopamine. Many of these hormones are released in bursts, and growth hormone has a large burst at the start of deep sleep in young adults. Doctors and scientists read the pituitary hormone and the gland hormone together, because the pair shows which part of the loop is not working.
College

Four anterior-pituitary axes share a plan: GHRH and somatostatin set GH, which acts directly and through hepatic IGF-1; dopamine tonically inhibits prolactin through D2 receptors; CRH, with vasopressin, drives ACTH and cortisol; TRH drives TSH and thyroid hormone. Long-loop feedback (target hormone to hypothalamus and pituitary) is well established in humans. Short-loop feedback (pituitary hormone to hypothalamus) has causal mouse evidence for prolactin and GH, and is less established for TSH and ACTH. Because secretion is pulsatile and circadian, a single sample is weak evidence. A paired trophic and target hormone locates a lesion: low target with high trophic hormone points to the gland (primary); low with low points to the pituitary or hypothalamus (secondary or tertiary), which the pair cannot separate. Suppression tests ask whether secretion is autonomous; stimulation tests ask about reserve.
Graduate

For a hormone cleared by first-order kinetics, C(t) = sum of A_i x 2^(-(t - t_i) / t-half) over pulses, so each pulse's contribution to one sample decays with the half-life: with GH half-lives measured at about 15 to 27 minutes, a sample mostly reflects the previous hour and can differ many-fold with a shift of tens of minutes. Integrated readouts (IGF-1, or a 24-hour mean from 10-minute sampling) trade timing for stability. Deconvolution of dense series separates secretion from clearance, which a concentration alone cannot. Measurement adds its own layer: immunoassays report immunoreactivity, LC-MS/MS reports a defined chemical species, and free hormone measured after equilibrium dialysis is the reference for the free fraction. Decision criteria are tied to assay: regression of newer specific cortisol assays against the older method gave lower stimulated criteria than the historical one. Open question: how much do short-loop arrows such as GH acting on GHRH neurons contribute to pulse generation in adult humans, given that the mouse evidence comes from lifelong, sex-dependent deletions and acute GH did not change GHRH-neuron firing in recordings?
Wake them, and the surge comes back

The 1968 sleep study did more than delay bedtime. The investigators also woke three of the young adult subjects for two to three hours in the night and then let them go back to sleep. On returning to sleep, the subjects showed another growth hormone peak. A clock-driven release would produce one surge at one hour whatever the subject was doing; a sleep-linked release should start again when sleep starts again. The second peak is what turned a correlation with night into evidence for a link with sleep onset itself, in these young adults.

— Takahashi, Kipnis and Daughaday, Growth hormone secretion during sleep, Journal of Clinical Investigation (1968).
The hormone the antibody sees and the receptor ignores

Some people carry much of their circulating prolactin as macroprolactin, a large complex of prolactin bound to an IgG antibody. Immunoassays can detect it, so the reported prolactin comes out high, but the complex has low biological activity. The high reading on the report is real, in the sense that the antibody did detect prolactin-like material, but how high it reads depends on the assay, and it is misleading about action at the receptor. The case is a clean example of the gap between immunoreactivity and bioactivity that every antibody-based hormone measurement carries.

— Haddad, Giacherio and Barkan, Interpretation of common endocrine laboratory tests: technical pitfalls, their mechanisms and practical considerations, Clinical Diabetes and Endocrinology (2019).
A criterion that moved with the assay

For many years, the cortisol value that labelled a response to synthetic ACTH as adequate was carried over from older, less specific immunoassays. In 2021 Javorsky and colleagues ran leftover serum from 110 patients tested this way in parallel on an older immunoassay and a newer one and, where volume allowed, on a second newer immunoassay and LC-MS/MS, and used regression to find the value on each method that matched the historical criterion. All three came out lower. Regression against an older method is not validation against outcomes, but it showed that a number written as a rule belonged to an assay.

— Javorsky and colleagues, New cutoffs for the biochemical diagnosis of adrenal insufficiency after ACTH stimulation using specific cortisol assays, Journal of the Endocrine Society (2021).

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