Endocrinology. L-3,5,3',5'-Tetraiodothyronine, T4. The major hormone derived from the thyroid gland. Thyroxine is synthesized via the iodination of tyrosine and the coupling of iodotyrosine in the thyroglobulin. Thyroxine is released from thyroglobulin by proteolysis and secreted into the blood. It is peripherally deiodinated to form triiodothyronine which exerts a broad spectrum of stimulatory effects on cell metabolism.