TRH

TRH is the hypothalamus's command to “turn up the thyroid.” It is a very small peptide — just three amino acids — that travels through the portal system to the anterior pituitary and prompts it to release TSH, which then drives the thyroid gland.

TRH sits at the top of the hypothalamic-pituitary-thyroid axis. When the body senses it needs more thyroid hormone — for instance, when exposed to cold — TRH output rises, lifting TSH and ultimately thyroid hormone production, which raises the body's metabolic rate and heat output.

Interestingly, TRH also stimulates the release of prolactin from the pituitary. This overlap helps explain why some people with severe primary hypothyroidism (high TRH because thyroid hormone is low) can also have mildly elevated prolactin and related symptoms.

The whole axis is governed by negative feedback: circulating thyroid hormones (T3 and T4) suppress both TRH and TSH. Measuring TSH is therefore the everyday clinical window onto this loop, while TRH itself is rarely measured directly.