Clinical physiology

The thyroid gland forms in the third week of gestation as an endodermal proliferation from the pharyngeal gut to which it remains attached by the thyroglossal duct. This subsequently fragments and disappears, although remnants may cause problems as thyroglossal cysts. The thyroid gland is made up of lobules composed of follicles. Their task is to synthesize, iodinate and store thyroglobulin and subsequently to reabsorb it, degrade the protein and release the hormones.

The steps in synthesis and release include:

1. iodide trapping by the thyroid gland;

2. synthesis of thyroglobulin in the follicular cells;

3. organification of trapped iodide as iodotyrosines;

4. coupling of the iodotyrosines to form iodothyronines;

5. storage of iodothyronines in extracellular colloid;

6. invagination and formation of intracellular colloid droplets;

7. hydrolysis of thyroglobulin to release iodotyrosines, T3 and T4;

8. deiodination of the iodotyrosines and recirculation of the iodide; release of the T3 and T4 into the circulation.

Some thyroglobulin escapes degradation and reaches the circulation through thyroid lymphatics in association with iodothyronines. Circulating thyroglobulin levels are high in the neonate and decrease in infancy and childhood. Concentrations increase when the thyroid gland is hyperactive, including after TSH administration; they decrease during thyroxine administration.

The thyroid gland is the sole source of T4. 70–90% of the T3 or reverse T3 in blood is derived from the monodeiodination of T4 by peripheral tissues.