Alterations of iodine metabolism in asymptomatic thyroiditis

Iodine metabolism has been studied in patients affected with asymptomatic auto-immune thyroiditis, as indicated by a significant titre of serum thyroglobulin antibodies, in the absence of overt thyroid disease. When matched with adequate controls, these subjects fail to show significant changes of 131I thyroid uptake, which was, however, markedly depressed by triiodothyronine and potassium iodide. The plasma PB 125I was increased. After a single injection of TSH, the iodine trapping was normally enhanced, but after 24 hours, the radioactive content of the thyroid was lower than in control subjects. It was shown that this phenomenon was related to the discharge from the thyroid of PBI with abnormally high specific activity. Kinetic studies indicated a 2 to 1 reduction of the thyroid exchangeable organic iodine, as well as of the thyroid 131I half life. The fraction of plasma PBI, extracted by butanol, was 73 ± 15 per cent in the thyroiditis patients, contrasting with a value of 93 ± 6 per cent in the controls. The iodine metabolism anomalies observed in asymptomatic thyroiditis may be explained by the reduction of the exchangeable iodine in the thyroid and its increased turnover. However, the interference of an underlying congenital metabolic defect cannot be excluded.