Thyroid hormones entry into cells

The phenomenon of thyroid hormone transport encompasses a number of sequential steps beginning with their secretion by the thyroid follicles and ending with their distribution to multiple sites of metabolism and action within virtually all the body s organs. These steps include equilibration with multiple carrier proteins in the plasma, transcapillary passage of both the hormones and the carrier proteins into extravas-cular spaces, entry into cells through their external membranes, and translocation to subcellular compartments and organelles. Some that are the subject of current inquiry or controversy, include the interaction of the hormones with "minor" transport protein in plasma, the kinetics of transcapillary movement, the presence of specific receptors for the carrier proteins and the hormones on endothelial and epithelial cell membranes, and specific transport mechanisms within cell organelles. 

T3 and T4 are stored in the follicular lumen bound to Tg. The re-entry of Tg into the thyroid follicular cell involves a macropinocytosis process. Thyroid hormones are released after proteolysis ofTg. 

The mechanism that determmcs the entry of the thyroid hormones into the cell and factors that control it are not clearly understood. Though the binding of the hormones to various components of the plasma proteins appears to be of major importance in regulating their passage across the cell wall, the ability of the various tissues to bind the hormones must play an equally important role. Otherwise, the marked variation in their concentration in different tissues would not exist. 

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