Hypothalamic-Pituitary Axes: Feedback Loops and Hormone Chains for the MCAT

Most endocrine MCAT questions are chains: hypothalamus releases a hormone, the anterior pituitary releases a tropic hormone, a target gland releases the final hormone, and that final hormone shuts the chain off by negative feedback. Know the chain and you can predict what rises or falls when one link fails.

The axes

Hypothalamic hormones travel to the anterior pituitary through the portal system. The posterior pituitary is different: it stores and releases ADH and oxytocin that the hypothalamus makes, and it is neural tissue.

Negative feedback

The final hormone inhibits both the pituitary and the hypothalamus. High thyroid hormone lowers TSH and TRH. High cortisol lowers ACTH and CRH. Prolactin is unusual because its hypothalamic control is inhibitory (dopamine), so cutting the connection raises prolactin. A few events use positive feedback, such as the LH surge from rising estrogen before ovulation, and oxytocin during labor.

Predicting disease: primary vs secondary

Find the broken link, then trace what each upstream hormone does.

• Primary hypothyroidism (thyroid failure): T4 low, so TSH rises (loss of feedback).
• Secondary hypothyroidism (pituitary failure): T4 low and TSH low or inappropriately normal.
• Cushing disease (ACTH-secreting pituitary tumor): ACTH high, cortisol high.
• Cortisol-secreting adrenal tumor: cortisol high, ACTH low (feedback suppresses the pituitary).
• Exogenous steroids: cortisol-like drug suppresses ACTH, and the adrenal cortex can shrink.

Worked example

A patient has low T4 and a high TSH. The thyroid is not responding even though the pituitary is working hard, so the defect is in the thyroid (primary). If TRH were given, TSH would rise further. In contrast, a patient with low T4 and low TSH whose TSH rises after TRH points to the hypothalamus (tertiary).

Common traps

• Forgetting the feedback direction. Low final hormone raises upstream hormones, unless the defect is upstream.
• Mixing up the pituitary lobes. ADH and oxytocin come from the posterior lobe but are made in the hypothalamus.
• Treating prolactin like the others. Dopamine inhibits it.
• Confusing peptide and steroid mechanisms. Tropic hormones are peptides acting through second messengers, and cortisol and sex hormones are steroids.

One-minute recap

Hypothalamus, anterior pituitary, target gland, final hormone, then negative feedback to the top. Thyroid: TRH, TSH, T3/T4. Adrenal: CRH, ACTH, cortisol. Gonadal: GnRH, LH/FSH, sex steroids. Primary failure raises upstream hormones, secondary failure lowers them.