Plasma membrane transport of thyroid hormones and its role in thyroid hormone metabolism and bioavailability

Although it was originally believed that thyroid hormones enter target cells by passive diffusion, it is now clear that cellular uptake is effected by carrier-mediated processes. Two stereospecific binding sites for each T-4 and T-3 have been detected in cell membranes and on intact cells from humans and other species. The apparent Michaelis-Menten values of the high-affinity, low-capacity binding sites for T-4 and T-3 are in the nanomolar range, whereas the apparent Michaelis-Menten values of the low-affinity, high-capacity binding sites are usually in the lower micromolar range. Cellular uptake of T-4 and T-3 by the high-affinity sites is energy, temperature, and often Na+ dependent and represents the translocation of thyroid hormone over the plasma membrane. Uptake by the low-affinity sites is not dependent on energy, temperature, and Na+ and represents binding of thyroid hormone to proteins associated with the plasma membrane. In rat erythrocytes and hepatocytes, T-3 plasma membrane carriers have been tentatively identified as proteins with apparent molecular masses of 52 and 55 kDa. In different cells, such as rat erythrocytes, pituitary cells, astrocytes, and mouse neuroblastoma cells, uptake of T-4 and T-3 appears to be mediated largely by system L or T amino acid transporters. Efflux of T-3 from different cell types is saturable, but saturable efflux of T-4 has not yet been demonstrated. Saturable uptake of T-4 and T-3 in the brain occurs both via the blood-brain barrier and the choroid plexus-cerebrospinal fluid barrier. Thyroid hormone uptake in the intact rat and human liver is ATP dependent and rate limiting for subsequent iodothyronine metabolism. In starvation and nonthyroidal illness in man, T-4 uptake in the liver is decreased, resulting in lowered plasma T-3 production. Inhibition of liver T-4 uptake in these conditions is explained by liver ATP depletion and increased concentrations of circulating inhibitors, such as 3-carboxy-4-methyl-5-propyl-2-furan-propanoic acid, indoxyl sulfate, nonesterified fatty acids, and bilirubin. Recently, several organic anion transporters and L type amino acid transporters have been shown to facilitate plasma membrane transport of thyroid hormone. Future research should be directed to elucidate which of these and possible other transporters are of physiological significance, and how they are regulated at the molecular level.