Thyroid hormone secretion, cell-membrane transport, metabolism and actionProper mediation of TH action requires: (1) the availability of intact TH; (2) its transport across the cell membrane; (3) intracellular metabolism and hormone activation; (4) cytosolic and nuclear processing; (5) association with receptors; and (6) interaction with co-regulators or other post-receptor effects required for the expression of TH effect.