Prolonged high iodine intake is associated with inhibition of type 2 deiodinase activity in pituitary and elevation of serum thyrotropin levels

Our previous epidemiological study indicated that excessive intake of iodine could potentially lead to hypothyroidism. In the present study, we aimed to investigate the time and dose effect of iodine intake on serum thyrotropin (thyroid-stimulating hormone, TSH) levels and to explore the non-autoimmune regulation of serum TSH by pituitary type 2 deiodinase (D2). A total of 360 Wistar rats were randomly divided into five groups depending on administered iodine dosages (folds of physiological dose): normal iodine (NI), 3-fold iodine (3HI), 6-fold iodine (6HI), 10-fold iodine (10HI) and 50-fold iodine (50HI). At 4, 8, 12 and 24 weeks after administration of sodium iodide, blood was collected for serum TSH measurement by chemiluminescent immunoassay. Pituitaries were also excised for measurement of TSHβ subunit expression, D2 expression and activity, monocarboxylate transporter 8 (MCT8) and thyroid hormone receptor β2 isoform (TRβ2) levels. The results showed that iodine intake of 10HI and 50HI significantly increased pituitary and serum TSH levels from 8 to 24 weeks (P?