Qualitative Changes in Thyroid Gland Secretion Following Thyroid Hormone Administration

The ratio of fecal to total excreted (fecal and urinary) radioiodine (F/T) was studied in mice with thyroid iodine labeled with 131I, and in those injected with mI-labeled thyroxine, triiodothyronine, monoiodotyrosine, diiodotyrosine or iodide. In the case of thyroid secretion of labeled components it was found that thyroid added to the diet or thyroid hormone injected led to the expected decrease in total 131I excretion, but in addition there was a fall in F/T from 0.5 to 0.2. Administration of thyroid did not appreciably affect the pattern of excretion of injected radio thyroxine (0.6) and triiodothyronine (0.5). The substituted tyrosines and iodide all gave an F/T of about 0.05. In vitro deiodinating activity (for 131I-diiodotyrosine) of the “suppressed” glands was markedly reduced in comparison with the activity of normal glands. It was postulated that this allowed the secretion of iodinated tyrosines, as well as thyronines, from the suppressed thyroid gland in vivo, which would account for the observed fall in F/T. (Endocrinology75: 554, 1964)