Reversible Inactivation of Thyrotropic Hormone by Elemental Iodine I. the Action of Iodine* by Alexander Albert,

The mechanism by which the thyrotropic or the thyroid-stimulating hormone (TSH) causes such marked growth and accelerated function of the thyroid remains a perplexing problem. Significant experiments in this direction have been reported by Rawson et al. (6, 8) who exposed TSH to slices of various organs in tissue culture. Of the various tissues studied, only thyroid, thymus, and lymph node were capable of inactivating an appreciable amount of hormone. In the case of thyroid tissue, it was observed that one rabbit thyroid weighing about 150 mg. was capable of inactivating 12 units of thyrotropic hormone. Furthermore, slices of human.thyroids obtained at operation varied in their ability to inactivate a given amount of hormone; i.e., slices of toxic goiters were twice as effective as equal amounts of normal tissue. Although the mechanism and meaning of this inactivation are obscure, one possibility is that in exerting its effect on the thyroid TSH may be partly metabolized. Such an inactivation could be carried out by an enzyme of the thyroid or by some substance formed by the thyroid (Rawson et al. (8)). The thyroid possesses the unique ability of,trapping circulating iodide and presumably converting it into iodine. Subsequently, the formation of thyroxine, which is the ultimate function of the thyroid,is possible. How TSH is integrated with this system is unknown, since the enzyme which supposedly converts iodide to iodine is purely theoretical and since no evidence is available showing any biochemical interrelation of TSH with any of the products of the thyroid. It would seem that the oxidation of iodide is a prime function of the thyroid. It is known that iodide has a depressing effect on the ability of concurrently administered TSH to induce goiters, and that iodide has an involuting action on the hyperplastic goiters of Graves’ disease and on the hyperplastic goiters induced by some antithyroid drugs.