Iodine contamination as a cause of hyperthyroidism or lack of TSH response to TRH stimulation (results based on a screening investigation).

The sera of all patients with completely suppressed TSH response to TRH obtained during one year (n = 668), and of those with diminished TSH response (n = 153) were screened for total serum iodine content. The ratio between serum iodine and thyroxine iodine below 1.5 indicates none or only a minor degree of iodine contamination, whereas a ratio above 1.5 is a clear index of exogenous iodine contamination. Eighty-four (21.3%) of 395 patients with overt hyperthyroidism were iodine contaminated. No prevalence of hyperthyroidism with hyperthyroxinemia could be detected as compared to T3-hyperthyroidism in the contaminated groups. Surprisingly, the iodine contamination rate was twice as high in 273 patients with suppressed TSH response to TRH but normal thyroid hormone levels and not fully explained thyroidal diseases. A high incidence of multifocal autonomous adenomas of the thyroid is the most probable explanation for the TSH suppression in iodine contaminated patients with normal thyroid hormone levels.