[Effect of mild and moderate excessive iodine supplementation on thyroid function and morphology in non-iodine deficiency rat model].

Objective: To study the effect of moderate and mild iodine excess on thyroid function and morphology in non iodine deficiency Wistar rats.

Methods: Serum thyroid-stimulating hormone (TSH) was measured with solid phase radioimmunoassay (IRMA) and serum total T(4) (TT(4)), TT(3), reverse T(3) (rT(3)) and thyroid TT(4) with radioimmunoassay (RIA). Cer-Arsenite colorimetric method was used to measure median urinary iodine (MUI). Thyroid morphological changes were observed under optical and electronic microscope. Metamorphic imaging system was used to measure the height of thyrocytes and the areas of thyroid follicular cavities.

Results: Iodine intake of three times than normal for 90 days could increase serum TSH but without significant difference; Serum TT(3) values were markedly lower than those in control, P = 0.0001. Serum and thyroid TT(4) values were markedly higher than those in double distilled water (DDW) group, P value being 0.001 and 0.0001 respectively. However, serum rT(3) did not change markedly when compared with that in DDW group. The area of follicular cavity increased, follicular cells became flat, nucleus stained darker, follicles broke and combined, giant follicles were formed, and the capillaries around the follicules were reduced. The height of thyrocytes was markedly lower than those in DDW group, while the areas of thyroid follicular cavities were markedly larger than those in DDW group (both P values were 0.001). Thyroid ultrastructure showed enlargement of endoplasmic reticulum of thyroid follicular cells, increase of number of secondary lysosomes, darker stain of nuclei, condensation of chromatin and reduction of microvilli.

Conclusion: Iodine intake over 3 times than normal may potentiate the possibility of hypothyroidism and inhibit the function of most of thyrocytes in non iodine deficiency rats.