Thyrotropin and Thyroglobulin as an Index of Optimal Iodine Intake: Correlation with Iodine Excretion of 39,913 Euthyroid Patients

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Abstract
The recommendations for the dietary allowance of iodine are 150 μg per day for adolescents and adults. Thyrotropin (TSH) and thyroglobulin (Tg) can be used as surveillance indicators for assessing iodine deficiency disorders. We compared the relation between TSH and Tg, free triiodothyronine, and thyroxine serum levels with urinary iodine excretion in 2311 untreated euthyroid patients using our modified cericarsenite method. An adequate iodine intake may be assumed when TSH and Tg values are at the lower end of the normal range. Patients were grouped according to urinary iodine excretion and goiter size. In the group with an iodine excretion between 201 and 300 μg of iodine per gram of creatinine, the lowest TSH values and even low Tg levels could be shown. We conclude that an iodine intake of approximately 250 μg/day is associated with the lowest TSH stimulation to thyrocytes. In the groups separated according to thyroid size, significantly higher Tg levels were found in the patients with uninodular and multinodular goiter as a result of longstanding iodine deficiency, whereas actual urinary iodine excretion did not differ significantly. Additionally, iodine excretion of 39,913 euthyroid patients between 1984 and 1996 was examined. In Austria iodized salt (10 mg KI/kg) was introduced by law in 1963 and increased to 20 mg KI/kg salt in 1990. An initial increase of iodine excretion until 1993 was followed by a decrease in 1994 and 1995 without further changes in 1996. These results show that iodine intake has improved since 1984; however, in 1996 iodine excretion in one-third of the investigated patients was under 100 μg per gram of creatinine and more than 80% had less than 200 μg per gram of creatinine.