Selenium and iodine supplementation: effect on thyroid function of older New Zealanders.

Randomized, double-blind trial in noninstitutionalized older adults aged 60 to 80 years living in Dunedin, New Zealand. Participants were generally free of serious illness and were not taking thyroid medications or selenium or iodine supplements.

Participants received oral tablets once daily for 12 weeks: 100 μg selenium as L-selenomethionine, 80 μg iodine, or the combined 100 μg selenium plus 80 μg iodine regimen. The selenium tablet contained a median 108 μg selenium, the iodine tablet 79 μg iodine, and the combined tablet 94 μg selenium plus 80 μg iodine; placebo contained <0.01 μg of selenium or iodine per tablet.

Selenium supplementation improved selenium status but did not improve thyroid hormone status, and iodine supplementation improved iodine status and lowered thyroglobulin but did not fully correct mild iodine deficiency over 12 weeks. Plasma selenium increased by week 12 in the selenium-containing groups, with unadjusted increases of 82% for selenium alone and 79% for selenium plus iodine; both were significant versus placebo after adjustment (P < 0.0001). Whole-blood GPx activity also increased with selenium alone by 4.5 U/g (95% CI: 2.7, 6.2) and with selenium plus iodine by 5.2 U/g (95% CI: 3.3, 7.1), both P < 0.0001 versus placebo. Urinary iodine concentration changed significantly only in the iodine group (P = 0.0014), and thyroglobulin fell by 27% with iodine and 18% with selenium plus iodine, with significance for iodine (P < 0.0001) and adjusted significance for the combined arm (P = 0.0053). There were no significant treatment effects for TSH, free T3, or free T4, and no evidence of synergy from combined selenium and iodine.

Each active arm was small (n = 25, 25, and 26), and the 12-week duration may have been too short to fully correct iodine deficiency or detect longer-term thyroid effects. The population had only mild deficiency and consisted of older adults from one region, which limits generalizability. No adverse events were reported, but the trial provides limited power to evaluate uncommon harms or subgroup effects.