Interaction of thyroid hormones with human muscular tissue, leucocytes and adipose tissue in vitro

The uptake of triiodothyronine (T3) and thyroxine (T4) by human leucocytes as well as by adipose and muscular tissues in vitro was markedly dependent on the extracellular protein concentration. The presence of extracellular protein was responsible for the difference in the uptake of T3 and T4 by leucocytes (the incorporation of T4 was significantly lower than that of T3), whereas its presence in the medium containing adipocytes did nto influence the incorporation values. In a protein-free medium, the muscular tissue uptake of T3 exceeded that of T4. In this tissue the deiodination of both hormones was likewise significantly different, that of T4 being higher. The conversion of T4 to T3 was also higher in the muscle cells, but represented only about 75% of the total deiodinated fraction. In the remaining tissues, no differences were observed in the uptake and deiodination of T3 and T4. The deiodination of T4 in human leucocytes was found to be considerably dependent of pH of the incubation medium (two deiodination optima, a higher one in acid region) whereas the deiodination of T3 was fairly indifferent to variations of pH (one flat deiodination optimum in the weakly alkaline region).