Hypothalamic-pituitary axes and feedback: primary, secondary, tertiary
Every hypothalamic-pituitary axis is a three-step chain held in check by negative feedback. Use that logic to predict which hormones rise or fall when the fault is in the gland, the pituitary or the hypothalamus.
Use negative-feedback logic of hypothalamic-pituitary axes to predict hormone levels in primary, secondary and tertiary disorders.
The short version
- Every axis is a three-step chain: hypothalamic releasing hormone, anterior pituitary trophic hormone, target gland hormone.
- The final hormone feeds back to lower the levels above it; that negative feedback is the logic of the whole topic.
- Primary means the target gland, secondary the pituitary, tertiary the hypothalamus.
- In primary disease the target and trophic hormones move in opposite directions; in secondary and tertiary disease they move in the same direction.
- Ask whether the trophic hormone is appropriate for the target hormone. Prolactin is the exception: it rises when the hypothalamic connection is lost.
The hypothalamic-pituitary axes: a three-step chain
A 41-year-old man has a free T4 below the reference range. A failing thyroid should push his TSH sky high, yet the lab reports that his TSH is low too. Where is the fault?
To answer, start with how a hypothalamic-pituitary axis is built. Every axis is a three-step chain. The hypothalamus sends a releasing hormone, the anterior pituitary answers with a trophic hormone, and the target gland makes the final hormone.[1]
The releasing hormones reach the pituitary through the hypophyseal portal system, a small capillary bridge in the stalk.[1][2]
Reading any axis therefore means reading three levels at once: the releasing hormone at the hypothalamus, the trophic hormone at the pituitary, and the target hormone from the gland. A fault at any one level changes the levels around it.[1]
| Hypothalamus | Anterior pituitary | Target gland |
|---|---|---|
| TRH | TSH | Thyroid |
| CRH | ACTH | Adrenal cortex |
| GnRH | LH and FSH | Gonads |
TSH, ACTH, LH and FSH are the trophic hormones: they stimulate the gland below them.[1][3]
Negative feedback: the logic of every axis
The final hormone switches down the levels above it. Thyroid hormone lowers TRH and TSH, cortisol lowers CRH and ACTH, and sex hormones lower GnRH, LH and FSH.[1][4]
Take the thyroid axis. When thyroid hormone is plentiful, TRH and TSH fall, so the thyroid is stimulated less. When thyroid hormone is scarce, TRH and TSH rise. The cortisol and sex-hormone axes work the same way.[1][4]
That is negative feedback, and it is the whole logic of this lesson. Remove the brake and the levels above rise; add a brake and they fall.
Primary, secondary and tertiary disorders: naming the level of the fault
The three terms name where the fault sits. Primary means the target gland itself. Secondary means the pituitary. Tertiary means the hypothalamus.[3][5]
Reading the pattern: opposite directions or the same direction
Primary disease. If the thyroid fails, thyroid hormone falls, the brakes come off, and TRH and TSH rise. Target hormone down, trophic hormone up: the two move in opposite directions.[3][6][7]
Secondary disease. If the pituitary is damaged, the thyrotroph cells cannot make TSH, so the thyroid is under-stimulated and thyroid hormone falls. Low thyroid hormone cannot rescue a pituitary that cannot respond, so TSH stays low, or sometimes inappropriately normal. Target hormone down, trophic hormone down: the same direction.[3][5][8]
Tertiary disease is a similar pattern one level up: TRH falls, so TSH usually falls, so thyroid hormone falls.[8]
| Level of the fault | Target hormone | Trophic hormone | Direction |
|---|---|---|---|
| Primary (gland) | Low | High | Opposite |
| Secondary (pituitary) | Low | Low, or inappropriately normal | Same |
| Tertiary (hypothalamus) | Low | Usually low, or inappropriately normal | Same |
- Free T4 is below the reference range: the target hormone is low
- A failing thyroid would push TSH up, but TSH is low
- Target low with trophic low: the same direction
- The fault is above the thyroid: pituitary or hypothalamus
A low free T4 with a low TSH points above the thyroid.[8]
Excess states run in reverse
A thyroid making too much hormone gives a high T4 and a low TSH: the brake is working. A TSH-secreting pituitary tumour gives a high T4 with a TSH that is not suppressed.[6][9]
The same question applies: is the trophic hormone appropriate for the target hormone? A raised T4 with a TSH that is not suppressed is not appropriate.
| Source of the excess | Thyroid hormone | TSH |
|---|---|---|
| Thyroid gland | High | Low |
| TSH-secreting pituitary tumour | High | Not suppressed |
A drug that mimics the hormone, and prolactin the exception
Feedback does not care where a hormone comes from. A drug that mimics the target hormone is read by the pituitary as the real thing.[4][10]
Prolactin is the one exception to memorise. It is held down by dopamine from the hypothalamus. Cut the stalk, and the other pituitary hormones fall, but prolactin rises, usually only mildly.[1][2]
Practice question
Original · USMLE styleA 54-year-old woman with rheumatoid arthritis comes to the clinic for a routine visit. She has taken prednisone daily for the past 10 months. She has gained weight around the abdomen and bruises easily. Examination shows a round face and thin skin on the forearms. She has no history of pituitary or adrenal disease. Assume the assays used measure only her own (endogenous) hormones.
Common questions
What is a hypothalamic-pituitary axis?
A three-step chain: the hypothalamus sends a releasing hormone, the anterior pituitary answers with a trophic hormone, and the target gland makes the final hormone. TRH to TSH to thyroid, CRH to ACTH to adrenal cortex, and GnRH to LH and FSH to gonads are the pairs to know.
What do primary, secondary and tertiary mean in endocrine disorders?
Primary means the target gland, secondary means the pituitary, and tertiary means the hypothalamus. Some hypopituitarism literature uses a different convention, so check which one a source follows.
How do hormone levels differ in primary and secondary disorders?
In primary disease the target hormone is down and the trophic hormone up: opposite directions. In secondary and tertiary disease both are down, or the trophic hormone is inappropriately normal: the same direction.
What does a low T4 with a low TSH suggest?
A fault above the thyroid, in the pituitary or hypothalamus. A failing thyroid itself would raise TSH.
Why does prolactin rise when the pituitary stalk is cut?
Dopamine from the hypothalamus normally holds prolactin down. Without that connection prolactin rises, usually only mildly, while the other pituitary hormones fall.
Transcript
Read the full narration, by chapter623 words
0:00The case: low free T4 with a low TSH, where is the fault?
A 41-year-old man has a free T4 below the reference range. A failing thyroid should push his TSH sky high. But the lab calls back: his TSH is low too. Where is the fault? Today: predict hormone levels when the fault is in the gland, the pituitary, or the hypothalamus.
0:20The three-step chain and the portal system
Every axis is a three-step chain. The hypothalamus sends a releasing hormone, the anterior pituitary answers with a trophic hormone, and the target gland makes the final hormone. The releasing hormones travel through the hypophyseal portal system, a small capillary bridge in the stalk.
0:39Axis pairs: TRH-TSH, CRH-ACTH, GnRH-LH/FSH
The pairs to know: TRH drives TSH, then the thyroid. CRH drives ACTH, then the adrenal cortex. GnRH drives LH and FSH, then the gonads.
0:55Negative feedback
The final hormone feeds back to switch down the levels above it: thyroid hormone lowers TRH and TSH, cortisol lowers CRH and ACTH, and sex hormones lower GnRH, LH and FSH. That is negative feedback, and it is the whole logic of this lesson.
1:17Primary, secondary and tertiary: naming and reading each pattern
Now name the level of the fault. Primary means the target gland itself. Secondary means the pituitary. Tertiary means the hypothalamus. Start with the thyroid. If the gland fails, thyroid hormone falls, the brakes come off, and TRH and TSH rise. Primary: target hormone down, trophic hormone up. They move in opposite directions. Now damage the pituitary. The thyrotroph cells cannot make TSH, so the thyroid is under-stimulated and thyroid hormone falls. Low thyroid hormone cannot rescue a pituitary that cannot respond, so TSH stays low, or sometimes inappropriately normal. Secondary: target hormone down, trophic hormone down. They move in the same direction. Tertiary is a similar pattern one level up: TRH falls, so TSH usually falls, so thyroid hormone falls. That solves our man: a low free T4 with a low TSH points above the thyroid.
2:26Excess states in reverse, and the one question to always ask
Excess runs in reverse. A thyroid making too much hormone gives high T4 and low TSH. ATSH-secreting pituitary tumour gives high T4 with a TSH that is not suppressed. Always ask: is the trophic hormone appropriate for the target hormone?
2:45A drug that mimics the hormone, and prolactin the exception
Feedback also does not care where a hormone comes from. A drug that mimics the target hormone is read by the pituitary as the real thing. One exception to memorise. Prolactin is held down by dopamine from the hypothalamus. Cut the stalk, and the other pituitary hormones fall, but prolactin rises, usually only mildly.
3:08Your turn: a practice question
Your turn. A 54-year-old woman with rheumatoid arthritis has taken prednisone every day for 10 months. She has a round face, central weight gain and easy bruising. Assume the assays measure only her own hormones. Which of the following best describes her morning ACTH and her own cortisol? A, ACTH high, cortisol high. B, ACTH high, cortisol low. C, ACTH low, cortisol low. D, ACTH low, cortisol high. E, both normal. Pause here and decide.
3:48The answer, one option at a time
A is out: high ACTH with high cortisol means something is driving the axis, as in a pituitary tumour. B is out: that is a failing gland, a primary pattern, and her adrenals are not diseased. D is out: low ACTH with high cortisol means the adrenal acts alone, but her cortisol is not high. E is out: prednisone is a steady brake on the axis, so it cannot stay normal. So the answer is C. Prednisone is read as cortisol, so it lowers CRH and ACTH, and her own adrenal makes little cortisol. Three things to keep.
4:34Recap pocket card
One: primary means the gland, secondary the pituitary, tertiary the hypothalamus. Two: in primary disease the two hormones move opposite ways; in secondary and tertiary they move together. Three: feedback cannot tell a pill from the real hormone, and prolactin is the exception that rises. Next, primary versus secondary disorders, with real pairs of lab results.
Sources
Numbers in the text, e.g. [1], point here- OpenStax Anatomy and Physiology 2e, 17.3 The Pituitary Gland and Hypothalamus. openstax.org/books/anatomy-and-physiology-2e/pages/17-3-the-pituitary-gland-and-hypothalamus
- Diagnosis and Treatment of Hypopituitarism. Endocrinol Metab 2015. europepmc.org/article/PMC/PMC4722397
- Metabolic Disorders and Male Hypogonadotropic Hypogonadism. europepmc.org/article/PMC/PMC6669361
- ESE/Endocrine Society guideline on glucocorticoid-induced adrenal insufficiency 2024. europepmc.org/article/PMC/PMC11180513
- Secondary adrenal insufficiency: an overlooked cause of hyponatremia. europepmc.org/article/PMC/PMC4330026
- Pitfalls in the measurement and interpretation of thyroid function tests. europepmc.org/article/PMC/PMC3857600
- Diagnosis of Primary Adrenal Insufficiency in Adults, JCEM 2023. europepmc.org/article/PMC/PMC10735307
- The diagnosis and management of central hypothyroidism in 2018. europepmc.org/article/PMC/PMC6373625
- Central hyperthyroidism due to an ectopic TSH-secreting pituitary tumor, Front Endocrinol 2024. europepmc.org/article/PMC/PMC10955116
- Iatrogenic Cushing case, Europe PMC 2026. europepmc.org/article/PMC/PMC13718705
The practice question is original and its patient is synthetic. Every fact is checked against the sources above in independent accuracy reviews. Always follow your local guidelines and your own laboratory’s ranges.
Image credits. Illustrations: Servier Medical Art (smart.servier.com), CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/), adapted (cropped, recoloured): pituitary, portal vessels, thyroid, adrenal, testis. Gray's Anatomy (1918), Wikimedia Commons, public domain. Pill icon by Marcel Tisch (bioicons.com), CC0. The diagrams, slips, cards and all handwriting are drawn in code (original). No AI-generated teaching images.
Corrections. None so far. Spotted an error? Email info@medservatory.com with the lesson code MS-S1-ENDO-009.
For exam study only, not medical advice. Read the medical disclaimer.
In this series
Playlist on YouTube- EP 01Hypothalamic-pituitary axes and feedback: primary, secondary, tertiary5:05
- EP 02Primary vs secondary endocrine disorders: paired hormone levels5:13
- EP 03Insulin secretion in the beta cell: from glucose to granule5:06
- EP 04Insulin vs glucagon signalling: receptors and effects5:19
- EP 05DKA vs HHS pathophysiology: why only one makes acid5:58
- EP 06Diabetes drugs by mechanism: where each class acts5:58
- EP 07Thyroid hormone synthesis, step by step5:14
- EP 08Calcium homeostasis: PTH, vitamin D and calcitonin5:28
- EP 09Adrenal steroidogenesis: zones, enzymes and the 17α-hydroxylase fork5:29
- EP 10Congenital adrenal hyperplasia: three enzymes, three patterns5:44
- EP 11Cushing syndrome: the dexamethasone suppression test and ACTH5:29
- EP 12Primary hyperaldosteronism: high aldosterone, low renin5:27
- EP 13SIADH vs diabetes insipidus: central vs nephrogenic DI5:44
- EP 14MEN1, MEN2A and MEN2B: match the gene to its tumours5:13