thyroxine

Iodine is incorporated into the tyrosine residues of thyroglobulin. Tyrosine is first iodized to monoiodotyrosine and then to diiodotyrosine, which later enzy­matically couple together and form thyi roxine and triiodothyronine. The synthe­sis and rel ease of thyroid hormones are controlled by the thyrotropin-releasing hormone—thyrotropin axis. Secretion of the thyrotropin-releasing hormone and thyrotropin is negatively regulated by thyroid hormone. The general effect of thyroid hormone is to activate transcrip­tion of nuclear target genes.