Thyroid basics
The thyroid is a butterfly-shaped gland in the neck, weighing around 25g, with each lobe being about 4cm long. Sitting just below the larynx (Adam’s apple), it typically has two sets of small pea-sized parathyroid glands attached to it.[1,2,3]
The main hormones secreted by the thyroid are thyroxine (T4, with four iodine atoms) and triiodothyronine (T3, the same molecular structure but with three iodine atoms) which are involved in controlling metabolism, growth and other functions.[1]
A multi-gland endocrine feedback system regulating thyroid activity originates in the base of the brain:[4]
1. The hypothalamus secretes thyrotropin-releasing hormone (TRH).
2. TRH stimulates the anterior pituitary to release thyroid-stimulating hormone (TSH) into the blood.
3. TSH binds to receptors on the thyroid stimulating thyroglobulin production and iodine uptake in the thyroid.
4. Enzymatic processes then lead to the production and release of T4 and T3 in the ratio 80% to 20%.
5. Circulating T3 and T4 inhibit TRH and TSH production by negative feedback which reduces iodine uptake and T3 and T4 secretion.
Small quantities of T2 and T1 thyronine are produced but their effects are still not fully understood. Another thyroid hormone is calcitonin, which acts with parathyroid hormone to control blood calcium levels.[1,4,5]