Studies with mouse pituitary thyrotropic tumors. X. Formation in vitro of 3,5,3'-triiodothyronine from thyroxine by dependent but not autonomous strains.

Labeled 3,5,3'-triiodothyronine (T3*) has been found in appreciable amounts in slices of transplanted dependent mouse pituitary thyrotropic tumors incubated in vitro with labeled thyroxine (T4*). On average, T3* constituted 18% of the total radioactivity in the tissue, whereas a maximum of 4% of the radioactivity was present as T3* in slices from a closely related autonomous strain. Tumor T3* concentrations in vivo after parenteral T4* administration, however, are high for both strains (1, 2). Slices from control tumors and tissues including boiled responsive tumor, incubated under the same conditions, contained 1-4% of the radioactivity as T3*. The difference in metabolic behavior in vitro between dependent and autonomous tumors may have a possible relationship to the mechanism of autonomy. (Endocrinology 79: 43, 1966)