Influence of Increasing Iodine Intake on Thyroid Function in Euthyroid and Hyperthyroid States*

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Abstract
This study was located in an area of iodine deficiencyin Western Germany inwhich incidental additional iodineintake occurs predominantly through iodine-containing x-raymedia and drugs. This offered the opportunity to investigate theinfluence of increasing iodine intake on the concentration ofthyroid hormones in 497 euthyroid individuals with and withoutgoiter and in 174 hyperthyroid patients with and without autonomousadenoma. In euthyroid subjects with and without goiter and a normalTRH test, the free T4 index(FT4I) increased, and T3 andmaximum TSH after TRH decreased significantly within theirnormal ranges in relation to increasing iodine intake. The resultssupport the concept of increased TSH secretion as well as theconcomitant alteration of theT3 to T4 ratio in iodine deficiencyand its normalization by increasing iodine intake within a rangeof iodine excretion from 10–1000 εg/g creatinine. In euthyroid individuals with and without goiterand a subnormalTRH test, the FT4I andT3 increased with increasingiodine intake. Compared to individual with a normal TRH test,the hormone concentrations were higher at all levels of iodineintake, in some cases reaching values above the upper normallimits at normal and supranormal iodine intakes. This mightincrease the frequency of iodine-induced hyperthyroidism andcan lead to diagnostic problems. In patients with hyperthyroidism not due to autonomousadenoma, the FT4I and T3increased rapidly by factors of 2.0 and 1.5, respectively, up to an iodineexcretiono 100-250 εg/gcreatinine. With a further increase in iodine intake, the FT4I andT3 decreased significantly without reaching the normal range. Inhyperthyroid patients with autonomous adenoma, these alterationswere muchsmaller. This proves quantitativel that hyperthyroidstates, especially in areas of iodinedeficiency, can deteriorateif iodine intake increases, and that hormone productionwill decrease if iodine intake further increases. Diagnostic problemscan result from T3 toxicosis, which is found more often withlow iodine intake,and from T4toxicosis, which occurs morefrequently after increased iodine intake. (J Clin EndocrinolMetab54: 1236, 1982)