Thyroid hormones are any hormones produced and released by the thyroid gland, namely triiodothyronine (T3) and thyroxine (T4). They are tyrosinebased hormones that are primarily responsible for regulation of metabolism. T3 and T4 are partially composed of iodine. A deficiency of iodine leads to decreased production of T3 and T4, enlarges the thyroid tissue and will cause the disease known as simple goitre.

The major form of thyroid hormone in the blood is thyroxine (T4), whose halflife of around one week[2] is longer than that of T3.[3] In humans, the ratio of T4 to T3 released into the blood is approximately 14:1.[4] T4 is converted to the active T3 (three to four times more potent than T4) within cells by deiodinases (5′deiodinase). These are further processed by decarboxylation and deiodination to produce iodothyronamine (T1a) and thyronamine (T0a). All three isoforms of the deiodinases are seleniumcontaining enzymes, thus dietary selenium is essential for T3 production.
Thyroid hormones (T4 and T3) are produced by the follicular cells of the thyroid gland and are regulated by TSH made by the thyrotropes of the anterior pituitary gland. The effects of T4 in vivo are mediated via T3 (T4 is converted to T3 in target tissues). T3 is three to five times as active than T4.

Thyroxine (3,5,3′,5′tetraiodothyronine) is produced by follicular cells of the thyroid gland. It is produced as the precursor thyroglobulin (this is not the same as thyroxinebinding globulin (TBG)), which is cleaved by enzymes to produce active T4.
The Na+/I− symporter transports two sodium ions across the basement membrane of the follicular cells along with an iodide ion. This is a secondary active transporter that utilises the concentration gradient of Na+ to move I− against its concentration gradient.
I− is moved across the apical membrane into the colloid of the follicle by pendrin .
Thyroperoxidase oxidizes two I− to form I2. Iodide is nonreactive, and only the more reactive iodine is required for the next step.
The thyroperoxidase iodinates the tyrosyl residues of the thyroglobulin within the colloid. The thyroglobulin was synthesised in the ER of the follicular cell and secreted into the colloid.
Iodinated Thyroglobulin binds megalin for endocytosis back into cell.
Thyroidstimulating hormone (TSH) released from the anterior pituitary (also known as the adenohypophysis) binds the TSH receptor (a Gs proteincoupled receptor) on the basolateral membrane of the cell and stimulates the endocytosis of the colloid.
The endocytosed vesicles fuse with the lysosomes of the follicular cell. The lysosomal enzymes cleave the T4 from the iodinated thyroglobulin.
The thyroid hormones cross the follicular cell membrane towards the blood vessels by an unknown mechanism.[28] Text books have stated that diffusion is the main means of transport,[31] but recent studies indicate that monocarboxylate transporter (MCT) 8 and 10 play major roles in the efflux of the thyroid hormones from the thyroid cells.