The predominant thyroid hormone released from the gland is T4, with lesser amounts of T3, a portion of which is formed by intrathyroidal deiodination of T4. The trans port of T4 and T3 across the basolateral membrane of the thyrocyte into the circulation was long assumed to take place by passive diffusion. More recently, however, it has become clear that the process is mediated by one or more transport proteins. The monocarboxylate trans port protein, MCT8 (Figure 1), is currently the prime candidate for this role in thyroid hormone secretion as it is expressed in the basolateral membrane of the thyrocyte and its absence in mice results in lowered secretion of thyroid hormone. This is the same transport protein that participates in bringing T3 and T4 into target cells, as discussed in section VI.A following. 
Fig1. Thyroid epithelial cell structure and function. In the two thyrocytes shown, the organelles (left) and proteins (right) involved in thyroid hormone synthesis and secretion are depicted. The arrows in the thyrocyte on the left indicate the pathway of thyrogobulin (Tg) from its synthesis in the endoplasmic reticulum through the Golgi apparatus; secretion, accompanied by thyroid hormone synthesis (Tg with T4, T3, MIT, and DIT), into the lumen; re-uptake into the cell at the microvilli (M) or pseudopods (P) to form a colloid droplet (CD); breakdown in lysosomes (Ly) and release of thyroid hormone into the capillaries. At the basal membrane of the thyrocyte on the right, are shown NIS, the sodium/iodide transporter that is responsible for iodide uptake into the thyrocyte; TSHR=receptor for thyroid stimulating hormone; and MCT8=monocarboxylate transporter 8, which participates in secretion of T4 and T3 into the capillaries. At the apical membrane are TPO, thyroid peroxidase, which catalyzes the steps of thyroid hormone synthesis as Tg moves into the lumen and the dual oxidase, DUOX, which supplies peroxide (H2O2) for the reactions. Pendrin, a solute carrier protein, may participate in the movement of I− from the cell into the lumen of the thyrocyte. Within the cell, unused I− is removed from MIT (monoiodotyrosine) and DIT (diiodotyrosine) by DEHAL, iodotyrosine dehalogenase.
The iodine in the MIT and DIT residues of thyroglobulin is released through the action of iodotyrosine dehalogenase (DEHAL1) and returned to the intrathyroidal I− pool. DEHAL is an NADPH-dependent flavoprotein which shows specificity for iodotyrosines relative to iodothyronines. Since the amount of iodide recaptured per day by the thyroid gland is 2–3 times the amount of “new” iodide transported into the gland from the blood, it is clear that the efficiency of this reutilization pathway is of fundamental importance to the overall iodine economy, and therefore, thyroid function.
