Endocrine physiology for FCPS Part 1
Endocrine questions test feedback loops, where each hormone comes from, how it signals and what its excess or deficiency looks like. Learn each axis as a loop.
On this page
How endocrine questions are built
Endocrine physiology is examined as loops. Each hormone has a stimulus, a source, a target, an effect and a feedback signal. Most questions cut the loop at one point and ask what happens to the others.
If a patient has a pituitary tumour making ACTH, what happens to cortisol, to CRH and to the other adrenal? If the adrenal itself is overactive, what happens to ACTH? Once you can trace the loop, you can answer it.
How hormones signal
| Receptor type | Examples |
|---|---|
| Gs (cAMP up) | β receptors, glucagon, TSH, ACTH, LH, FSH, PTH, V2 |
| Gi (cAMP down) | α2, M2, somatostatin |
| Gq (IP₃ and calcium) | α1, V1, angiotensin II, GnRH, TRH, oxytocin |
| Tyrosine kinase | Insulin, IGF-1 |
| JAK–STAT | Growth hormone, prolactin, erythropoietin |
| Nuclear receptor | Steroids, thyroid hormone, vitamin D |
Thyroid hormone acts on a nuclear receptor, like the steroids, even though it is made from an amino acid.
The pituitary
- Anterior pituitary (from Rathke pouch): GH, prolactin, ACTH, TSH, LH and FSH.
- Posterior pituitary (neural): stores ADH and oxytocin made in the hypothalamus.
- Prolactin is the only anterior pituitary hormone under mainly inhibitory control, by dopamine. That is why stalk compression and dopamine-blocking drugs raise prolactin.
- Growth hormone acts largely through IGF-1 from the liver. It is released in pulses, especially in deep sleep, and suppressed by glucose (the basis of the glucose suppression test for acromegaly).
Thyroid
- Iodide is trapped by the sodium–iodide symporter, then thyroid peroxidase organifies and couples it. Carbimazole and propylthiouracil block this enzyme.
- The gland releases mostly T4. Peripheral tissues convert it to the more active T3.
- Most hormone in blood is bound to thyroxine-binding globulin. Pregnancy and oestrogen raise TBG, so total T4 rises while free T4 stays normal.
- TSH is the best single screening test for thyroid function.
Adrenal
| Zone | Hormone | Main control |
|---|---|---|
| Glomerulosa | Aldosterone | Angiotensin II and potassium |
| Fasciculata | Cortisol | ACTH |
| Reticularis | Androgens | ACTH |
| Medulla | Adrenaline, noradrenaline | Sympathetic nerves; cortisol induces the enzyme PNMT |
- Cortisol peaks in the early morning and is lowest around midnight.
- In primary adrenal failure (Addison disease), ACTH is high, which darkens the skin, and aldosterone is also low, so potassium rises.
- In secondary adrenal failure (pituitary disease), ACTH is low, there is no pigmentation, and aldosterone is largely preserved.
Calcium
- PTH raises calcium by increasing bone resorption, increasing distal tubular calcium reabsorption, reducing phosphate reabsorption and activating vitamin D in the kidney.
- Calcitriol (1,25-dihydroxyvitamin D) raises intestinal calcium and phosphate absorption.
- Calcitonin from thyroid C cells lowers calcium, but has little role in day-to-day control.
- Low magnesium blocks PTH release and action, so hypocalcaemia will not correct until magnesium is replaced.
Glucose
| Hormone | Effect on glucose | Main actions |
|---|---|---|
| Insulin | Lowers | GLUT4 in muscle and fat, glycogen synthesis, lipogenesis, potassium into cells |
| Glucagon | Raises | Glycogenolysis and gluconeogenesis in the liver |
| Adrenaline, cortisol, GH | Raise | Counter-regulatory hormones |
- Glucose enters the beta cell through GLUT2, ATP rises, K-ATP channels close, the cell depolarises and calcium triggers insulin release. Sulphonylureas close the same channels.
- C-peptide is released with endogenous insulin. It is low when hypoglycaemia is caused by injected insulin.
Common traps
- ADH acts through V2 (Gs) in the kidney and V1 (Gq) on blood vessels.
- In primary hyperaldosteronism, aldosterone is high and renin is low. In renal artery stenosis both are high.
- In 21-hydroxylase deficiency, cortisol and aldosterone are low, androgens high, so girls are virilised and infants may lose salt.
- Oxytocin and the LH surge are the classic positive feedback loops.
How to revise in one week
- Day 1: hormone types, receptors and second messengers.
- Day 2: hypothalamus and pituitary, GH and prolactin.
- Day 3: thyroid.
- Day 4: adrenal cortex and medulla.
- Day 5: calcium, PTH and vitamin D.
- Days 6 to 7: pancreas and glucose control, then mixed questions.
Practise these in the FCPS-1 Prep question bank. Endocrine questions link to topic reviews that trace each loop from hypothalamus to target tissue.
Read next: Physiology high-yield · Biochemistry high-yield · Pharmacology high-yield
Frequently asked questions
Which endocrine topic is tested most?
Thyroid and adrenal physiology, followed by calcium control and the second messenger of each hormone.
Do I need to know disease management?
For Part 1, focus on the physiology and the hormone patterns in disease. Detailed treatment matters more in later exams.
How do I remember second messengers?
Learn the short Gq and Gi lists. Most other peptide hormones that act on the cell surface use Gs and cAMP.