Thiocyanate feeding with low iodine diet causes chronic iodine retention in thyroids of mice

Effects of KSCN on thyroidal iodine metabolism were studied in weanling mice fed a low iodide diet (LID) labeled continuously with 125I as iodide. The addition of KSCN (0.3 and 0.6 mg/g diet) resulted in the accumulation of an unusual iodinated protein within the follicles of the mouse thyroids. After 60 days, total thyroidal iodine was 4 times greater than in controls without thiocyanate. The iodinated protein was essentially insoluble at pH 8.0 and was very slowly released from the thyroids; it contained more MIT than DIT and little thyroid hormone. By use of three isotopes (125I, 127I, and 131I) and auto-radiographs, there were shown different iodinated proteins synthesized during high and low iodine intakes and coexistent but segregated in the colloid. Low doses of perchlorate or iodide inhibited or prevented accumulation of the essentially insoluble iodinated protein. It is suggested that when mouse thyroids are iodine depleted, thiocyanate increases the formation of an essentially insoluble iodinated thyroglobulin within the thyroid. Only a small fraction of the iodination may have occurred by this route, but the rate of formation exceeded the rate of release, so the product continuously accumulated.