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Effects of selenomethionine supplementation on selenium status and thyroid hormone concentrations in healthy adults^1,2,3,4

¹ From the Grand Forks Human Nutrition Research Center, US Department of Agriculture, Agricultural Research Service, Grand Forks, ND (GFC and WKC); the Division of Nutritional Sciences, Cornell University, Ithaca, NY (GFC); the Division of Cancer Prevention, National Cancer Institute, Rockville, MD (DNM, PRT, and BHP); the Beltsville Human Nutrition Research Center, US Department of Agriculture, Agricultural Research Service, Beltsville, MD (KYP, ADH, and OAL); and Information Management Services, Inc, Rockville, MD (JEM).

² ADH is deceased.

³ Supported by the Intramural Research Program of the NIH, the National Cancer Institute, and the Division of Cancer Epidemiology and Genetics through an interagency agreement with the Agricultural Research Service and a specific cooperative agreement with Cornell University.

⁴ Address correspondence to GF Combs, Grand Forks Human Nutrition Research Center, 2420 2nd Avenue N, Grand Forks, ND 55202. E-mail: gerald.combs{at}ars.usda.gov.

Background: Selenium, a potential cancer prevention agent currently being tested against prostate cancer in the Selenium and Vitamin E Cancer Prevention Trial (SELECT), plays an integral role in thyroid metabolism. The effects of long-term selenium supplementation on thyroid hormone concentrations are unknown.

Objective: The objective was to investigate the effects of long-term selenium supplementation on thyroid hormone concentrations.

Design: Twenty-eight healthy adults took 200 µg selenomethionine/d for 28 mo. The thyroid hormones triiodothyronine (T₃), thyroxine (T₄), and thyrotropin (TSH) were measured in plasma for 4 mo before supplementation and quarterly during supplementation. The assay methods were changed midstudy; the results of the 2 methods were not comparable. Therefore, one analysis was conducted based on the results of the first method, and a second analysis was based on all of the data, adjusted for the change. Serial data collection permitted a test for trends rather than simply a difference between initial and final values.

Results: By 9 mo, mean (±SEM) plasma selenium concentrations had increased from 1.78 ± 0.07 µmol/L at baseline to 2.85 ± 0.11 µmol/L for men and from 1.64 ± 0.04 to 3.32 ± 0.1.2 µmol/L for women. T₃ concentrations in men increased 5% per year (P = 0.01). T₄ and TSH concentrations were unchanged.

Conclusions: Selenium supplementation produced no clinically significant changes in thyroid hormone concentrations. A small but statistically significant increase in T₃ concentrations was noted in men, with no corresponding decreases in TSH. A subset of SELECT subjects might be monitored periodically for changes during long-term selenium supplementation.

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