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

The New Zealand population has both marginal selenium status and mild iodine deficiency. Adequate intakes of iodine and selenium are required for optimal thyroid function.

The aim of the study was to determine whether low selenium and iodine status compromises thyroid function in an older New Zealand population.

We investigated the effects of selenium and iodine supplementation in a double-blind, randomized, placebo-controlled trial in 100 Dunedin volunteers aged 60-80 y. Participants received 100 microg Se/d as l-selenomethionine, 80 microg I, 100 microg Se + 80 microg I, or placebo for 3 mo. Thyroid-stimulating hormone (TSH), free triiodothyronine (T(3)), free thyroxine (T(4)), thyroglobulin, plasma selenium, whole-blood glutathione peroxidase (GPx) activity, and urinary iodine concentrations (UICs) were measured.

Plasma selenium (P < 0.0001) and whole-blood GPx activity (P<0.0001) increased from baseline to week 12 in the selenium and selenium plus iodine groups in comparison with the placebo group. Median UIC at baseline was 48 microg/L (interquartile range: 31-79 microg/L), which is indicative of moderate iodine deficiency. UIC increased in the iodine and selenium plus iodine groups and was significant only for the iodine group (P = 0.0014). Thyroglobulin concentration decreased by 24% and 13% of baseline in the iodine and selenium plus iodine groups in comparison with the placebo group (P = 0.009 and P = 0.108, respectively). No significant treatment effects were found for TSH, free T(3), free T(4), or ratio of T(3) to T(4).

Additional selenium improved GPx activity but not the thyroid hormone status of older New Zealanders. Iodine supplementation alleviated the moderate iodine deficiency and reduced elevated thyroglobulin concentrations. No synergistic action of selenium and iodine was observed. The trial was registered at www.anzctr.org.au/registry/ as ACTRN012605000368639.