What the thyroid gland makes
The thyroid gland produces two hormones: thyroxine (T4) and triiodothyronine (T3). They act on cells throughout the body, and they do so through thyroid hormone receptors.
One loop
Making two hormones is one part of the story, and the same hormones also feed back on the axis that sets how much the gland makes. The output of the thyroid is regulated by the hypothalamic-pituitary-thyroid axis. The gland does not set its own output. The hypothalamus releases thyrotropin-releasing hormone (TRH), the anterior pituitary responds with thyroid-stimulating hormone (TSH), and TSH drives the thyroid.
The hormone then goes back to those same control centres. Thyroid hormones restrain TSH and TRH by negative feedback. Thyroid physiology is one chain: a gland with a particular structure, controlled by TSH, makes and delivers hormone, and the hormone acts on cells.
The Thyroid gland structure, development and control note shows how the brain sets the gland, through TRH and TSH, and how the hormone feeds back to them.
The parts of the loop
The loop has several parts, and each satellite explains one of them. The hormone’s actions shape development and adult tissues, and pregnancy changes the whole system.
- Thyroid gland structure, development and control: where the gland sits, how it is built, how it forms in the embryo, and how TSH from the anterior pituitary drives and restrains the whole axis.
- Thyroid hormone metabolism and action: how the gland takes up iodine and builds T4 and T3, how the hormones are carried in blood and converted to active T3, and how they act inside cells.
- Effects of thyroid hormone on development and adult tissues: how the hormone affects the developing brain and body, and what it affects after birth for energy use, the heart, the liver, the nervous system and reproduction.
- Thyroid function in pregnancy: how pregnancy changes the thyroid axis, and the normal ranges and the hormone requirement in pregnancy.
Where to start
Start with the first part, which describes the gland itself. Begin with Thyroid gland structure, development and control. So where does the chain begin? In the gland itself, with where it sits, how it is built, and what drives it.