Transport of iodothyronines into tissues

Plasma membrane transport of thyroid hormone into rat hepatocytes 

Na+ gradient across the cell membrane, as evidenced by the effects of the Na+,K+-ATPase inhibitor ouabain [104]. 

The concept that plasma membrane transport plays a key role in the regulation of intracellular thyroid hormone levels is supported by studies with a monoclonal antibody against an antigen exposed on rat liver cells [107], This antibody inhibited the uptake of different iodothyronines by rat hepatocytes under initial rate conditions as well as the metabolism of these compounds during prolonged incubations [107]. Uptake and metabolism of T4, T3 and rT3 were affected to the same extent, suggesting that a single system operates in the transport of different iodothyronines, which is opposite to the view advanced above. However, it is not excluded that the antibody interacts with a component of the plasma membrane and thereby affects multiple transport systems. 

Energy-dependent, carrier-mediated uptake of thyroid hormone has been demonstrated in a variety of other cell types, including fibroblasts [109,110], pituitary tumor cells [111] and muscle cells [112]. In addition to the uptake processes in the plasma membrane, indirect evidence has been reported recently for active transport of T3 between subcellular compartments, i.e., from the cytoplasm to the nu- 

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