Lecture 2: Thyroid Physiology

The hypophyseal portal system is a capillary-vein-capillary network running in the pituitary stalk: superior hypophyseal arteries form a primary capillary plexus in the median eminence, receiving hypothalamic releasing/inhibiting hormones (TRH, CRH, GnRH, GHRH, dopamine, somatostatin); these drain into the long portal veins, then into a secondary capillary plexus in the anterior pituitary where hormones act on target cells in concentrations ~1000× systemic. Stalk injury disconnects this supply → hypopituitarism with one exception: dopamine (PIH) inhibition of lactotrophs is also lost, so prolactin rises (‘stalk-effect’ hyperprolactinemia). The posterior pituitary, by contrast, receives direct axonal projections, not portal blood.

The posterior pituitary (neurohypophysis) is a neural extension of the hypothalamus. Magnocellular neurons in the supraoptic and paraventricular nuclei send their axons down the pituitary stalk to terminate in the posterior pituitary, where they release ADH and oxytocin directly into systemic circulation. The posterior pituitary has no portal blood supply and does not synthesize its own hormones — it simply stores and releases what the hypothalamus delivers. Damage to these axons (trauma, surgery, tumor, autoimmune hypophysitis) causes central DI. This hypothalamus–neurohypophysis axonal projection is a defining anatomical and functional connection in LO 1.

Oxytocin and ADH are synthesized by magnocellular neurons in the supraoptic (SON) and paraventricular (PVN) nuclei of the hypothalamus and transported via axons to the posterior pituitary, where they are released into systemic circulation. The PVN primarily makes oxytocin (milk let-down, uterine contraction during labor); the SON primarily makes ADH. The arcuate nucleus makes GnRH and GHRH; the ventromedial nucleus is the ‘satiety center’; the suprachiasmatic nucleus is the master circadian clock; the preoptic nucleus regulates thermoregulation and gonadotropin release. Mapping hypothalamic nucleus → hormone is classic LO 2 material.

In Kallmann syndrome, GnRH neurons fail to migrate from the olfactory placode to the arcuate nucleus, producing hypogonadotropic hypogonadism (↓GnRH → ↓LH/FSH → ↓sex steroids) plus anosmia. The arcuate nucleus is the origin of the GnRH pulse generator — pulsatile GnRH release (every ~90 min) drives LH and FSH secretion. The arcuate also produces GHRH and dopamine (PIH). Understanding which nucleus produces which hormone is exactly what LO 2 asks; Kallmann is the classic clinical corollary.

The adenohypophysis develops from Rathke’s pouch, an evagination of oral ectoderm. On H&E, its hormone-secreting cells are classified by staining: acidophils (somatotrophs → GH; lactotrophs → prolactin), basophils (corticotrophs → ACTH; thyrotrophs → TSH; gonadotrophs → LH/FSH), and chromophobes (mostly nonsecretory or degranulated). Craniopharyngiomas arise from Rathke’s pouch remnants — that’s why they sit suprasellar. The neurohypophysis, by contrast, is neural ectoderm: pituicytes + unmyelinated axon terminals storing ADH/oxytocin. Distinguishing the two embryologic origins is core LO 3 content.

Acromegaly from excess GH points to a somatotroph adenoma. Somatotrophs are acidophilic and constitute the most abundant anterior pituitary cell (~50%) — which is why GH adenomas are the second most common secretory adenoma (after prolactinomas, which also arise from acidophils, ~15–20%). Approximate proportions to know: somatotrophs ~50%, lactotrophs ~15–20%, corticotrophs ~15–20%, gonadotrophs ~10%, thyrotrophs ~5%. LO 3 emphasizes mapping histologic appearance + approximate cell composition to function.

Physiologic hypothalamic regulation of the gonadotrophs requires pulsatile GnRH release (~1 pulse / 90 min from the arcuate nucleus). Continuous exposure to a GnRH agonist causes an initial LH/FSH flare followed by desensitization and down-regulation of gonadotroph GnRH receptors → suppressed LH/FSH → suppressed sex steroids. This is used clinically for precocious puberty, prostate cancer, endometriosis, and uterine fibroids. The opposite (pulsatile GnRH infusion via pump) is used to restore fertility in hypothalamic amenorrhea. Pattern of hypothalamic signaling — tonic vs pulsatile — is a central LO 4 concept.

Prolactin is unique among anterior pituitary hormones in being under tonic hypothalamic inhibition by dopamine (prolactin-inhibiting hormone, PIH) acting on lactotroph D2 receptors. When the pituitary stalk is disrupted (trauma, large macroadenoma, stalk mass effect), dopamine no longer reaches the anterior pituitary → lactotrophs are disinhibited → prolactin rises (‘stalk effect’); other hormones fall because their hypothalamic stimulators (TRH, CRH, GnRH, GHRH) can no longer reach them. This is also why D2 agonists (cabergoline, bromocriptine) treat prolactinomas. Tonic inhibition is the signature hypothalamic regulation of lactotrophs in LO 4.

The HPA axis is governed by negative feedback: cortisol (or exogenous glucocorticoids like prednisone) inhibits hypothalamic CRH and pituitary ACTH. Chronic suppression leads to adrenal cortex atrophy over weeks to months. When the stressed axis cannot mount its normal cortisol surge, the patient is at risk for adrenal crisis. Stress-dose glucocorticoids are required perioperatively; abrupt steroid withdrawal can also precipitate crisis. This is the signature clinical application of LO 5 feedback physiology.

Most endocrine feedback is negative (HPT, HPA, cortisol/aldo on target organs). The mid-cycle LH surge is one of the few clear examples of positive feedback in endocrinology: rising estradiol from the dominant follicle, once it crosses the threshold and is sustained, switches from mild negative feedback to robust positive feedback on the hypothalamus (kisspeptin/GnRH) and pituitary (gonadotroph LH stores) → surge → ovulation ~36 hours later. (The other classic positive-feedback example is oxytocin during labor.) LO 5 requires you to name the loop type and trace its origin and target.

The thyroid gland is the target organ of the HPT axis. Iodine deficiency ↓ T3/T4 synthesis → loss of negative feedback on hypothalamus and pituitary → ↑TRH → ↑TSH → chronic TSH stimulation of follicular cells → hypertrophy/hyperplasia → goiter. This is the canonical ‘↑↑TSH + ↓T4 = primary hypothyroidism’ pattern that end02 LO 6 asks you to predict when the target organ fails. Secondary (↓TSH + ↓T4) would require pituitary failure; TSH-oma would produce ↑TSH + ↑T4; autonomous nodule would suppress TSH.

End02 LO 6 asks you to predict hormone iterations at each level of the axis. ↑T4 with an inappropriately normal TSH is the classic ‘inappropriate TSH’ pattern. Generalized resistance to thyroid hormone (TR-β mutation): the pituitary’s TR-β cannot sense high T4 → negative feedback fails → TSH remains normal or slightly elevated even with high T4. Peripheral tissues with normal TR-α (e.g., heart) still respond to circulating T3 → symptoms. A TSH-oma would give the same axis pattern but usually shows a pituitary mass on MRI. In Graves or factitious T4 ingestion, negative feedback would suppress TSH toward zero. Primary hypothyroidism shows ↑TSH + ↓T4 (the opposite).

↑↑TSH + ↓free T4 + positive anti-TPO = primary hypothyroidism from Hashimoto thyroiditis (autoimmune destruction of the thyroid gland). The pituitary is working hard to compensate → TSH is very high. Secondary (pituitary) hypothyroidism would give ↓TSH + ↓T4 (the pituitary cannot drive the gland). Tertiary (hypothalamic) would give ↓TRH, ↓TSH, ↓T4. Subclinical hypothyroidism shows ↑TSH with a normal free T4 (earlier stage of the same disease spectrum). The axis iterations of end02 LO 6 let you identify the defective level from labs alone.

↓TSH + ↓free T4 is the classic secondary (central) hypothyroidism pattern: the pituitary cannot produce TSH, so the thyroid is not stimulated and T4 falls. A craniopharyngioma and its resection are well-known causes of pituitary dysfunction (damage to thyrotropes). Treat with levothyroxine. Cannot use TSH to monitor adequacy of replacement, because the pituitary itself is broken — monitor free T4 instead. Primary hypothyroidism would show ↑TSH + ↓T4 (opposite). This question also links to LO 3 (pituitary anatomy/histology) — craniopharyngiomas damage the adenohypophysis, where thyrotropes live.

Graves disease is driven by TSI, an IgG autoantibody that binds and constitutively activates the TSH receptor, bypassing the pituitary. The thyroid behaves as if it had a permanent TSH signal → ↑↑T4 → negative feedback suppresses hypothalamic TRH and pituitary TSH → TSH falls to ~0.01. The axis iteration is ↓↓TRH, ↓↓TSH, ↑↑T4. Note that ‘primary hyperthyroidism’ in end02 LO 6’s iteration table also gives ↓TSH + ↑T4, regardless of whether the thyroid is hyper-driven by TSI, a toxic nodule (somatic TSHR mutation), or exogenous T4 — negative feedback works normally.