Effect of chronic iodide supplements on thyroid secretion: the role of lysosomes.

Abstract

The role of thyroid lysosomes in hormonal secretion has been studied in rats fed a low iodine diet and receiving chronic increasing iodide supplements. Thyroid secretion was assessed by plasma TSH, T3, and T4 levels and by the concentration and degree of iodination of thyroglobulin (Tgb). Lysosomal activity was evaluated by the total activity of four lysosomal acid hydrolases and by the integrity and fragility of the lysosomal membrane, which were assessed by the latency of acid phosphatase activity and the susceptibility of the particles to physicochemical manipulations. In glands from animals on a low iodine intake and under chronic TSH hyperstimulation, the integrity of the lysosomes was modified (increased free, but sedimentable, acid phosphatase activity), and the lysosomal membrane cohesion was high (increased resistance to freezingthawing and acid autolysis). In animals receiving increasing physiological iodide supplements, plasma TSH levels were normal, and the size of the intrathyroidal iodine pool autoregulated thyroid secretion; in spite of the increased concentration and iodine content of Tgb, the plasma levels of T3 and T4 remained fairly constant. The total activities of the four lysosomal enzymes tested increased gradually with the iodide supply. The integrity and stability of the lysosomes were not modified. Iodine at high pharmacological doses was not a lysosome stabilizer but induced a labilization of the lysosomal membrane. We suggest that lysosomes may participate in the autoregulation of thyroid secretion by the intrathyroidal iodine pool; the synthesis of new lysosomal hydrolases is induced, which may gradually be involved in the degradation of cellular constituents rather than in the hydrolysis of extracellular Tgb.