Thyroid hormone release involves the proteolysis of thyroglobulin.

Thyroglobulin hydrolysis products include Thyroxine (T4) and tri-iodothyronin (T3).

Other iodinated tyrosine residues are deiodinated within the gland and the iodine reutilized.

T3 and T4 are released by exocytosis.

The thyroglobulin T4 /T3 ratio is equal to 5:1, thus T4 (thyroxine) is the primarily hormone that is released in the circulation.

Above reactions may be inhibited by chemical agents, collectively termed goitrogens (they tend to produce goitre - enlargement of the thyroid gland through stimulation of TSH secretion -

Examples:
perchlorate and thiocyanate-- inhibit iodide transport
thiourea derivatives: inhibit iodide oxidation; decrease DIT/MIT ratios; block coupling of iodotyrosines to active iodothyronines