Labeled sulfur uptake by thyroids of rats with low plasma thiocyanate levels.

In a previous paper (1) we reported data, based upon the use of radiosulfur, showing that in the rat the affinity of the whole thyroid for thiocyanate ion is slight but is significantly greater than in other tissues. Baumann and Metzger (2), using different analytical procedures, reported a similar observation. The uptake of thiocyanate ion by the thyroid is of especial interest because of the chemical resemblance of the ion to iodide (3), as well as the physiological antagonism to iodide which thiocyanate exhibits. Thiocyanate is a goitrogen whose effect is reversed by iodide administration (4); it acts on the thyroid to release iodide from the gland, and it interferes with the formation of protein-bound iodine (5, 6). The metabolism of thiocyanate by the‘ thyroid is inhibited by propylthiouracil (1). From such considerations the question has been raised by several investigators (1, 5-9) whether thiocyanate might be accumulated by the thyroid to the degree or in the manner by which iodide is concentrated. The present study was made because the available data were inconclusive on these points. It has been observed that raising the iodide level of the plasma obscures the normal thyroid-serum iodine gradient, since the capacity of the gland to absorb iodide is limited (10). By analogy the thyroid might be similarly limited in its ability to absorb thiocyanate. This would explain the low concentration gradients in experiments previously reported (1, 2), since the large dosages used produced high plasma levels. It thus seems desirable to investigate the uptake of thiocyanate by the thyroid from very low plasma levels of this ion. Adult, male rats were prepared by feeding for 33 days a diet of standard laboratory ration, a ‘<low iodine” diet (ll), or a propylthiouracil diet. About 0.75 PM of thiocyanate ion labeled with S35 was injected into each animal and, after varying intervals of time, the ratios of labeled sulfur between the thyroids and equal weights of serum were determined by radioactivity measurements. The results are shown in Table I. In none