Does selenium supplementation affect thyroid function? Results from a randomized, controlled, double-blinded trial in a Danish population

Eur J Endocrinol. 2015 Jun;172(6):657-67. doi: 10.1530/EJE-15-0069. Epub 2015 Mar 4.


Objective: Selenium is present in the active site of proteins important for thyroid hormone synthesis and metabolism. The objective of this study is to investigate the effect of selenium supplementation in different doses on thyroid function, under conditions of suboptimal dietary selenium intake.

Design: The Danish PREvention of Cancer by Intervention with SElenium pilot study (DK-PRECISE) is a randomized, double-blinded, placebo-controlled trial. A total of 491 males and females aged 60-74 years were randomized to 100 μg (n=124), 200 μg (n=122), or 300 μg (n=119) selenium-enriched yeast or matching yeast-based placebo tablets (n=126). A total of 361 participants, equally distributed across treatment groups, completed the 5-year intervention period.

Methods: Plasma samples were analyzed for selenium and serum samples for TSH, free triiodothyronine (FT3), and free thyroxine (FT4) at baseline, and after 6 months, and 5 years of supplementation.

Results: Plasma selenium concentrations increased significantly and dose-dependently in treatment groups receiving selenium (P<0.001). Serum TSH and FT4 concentrations decreased significantly and dose-dependently by 0.066 mIU/l (P=0.010) and 0.11 pmol/l (P=0.015), respectively, per 100 μg/day increase, with insignificant differences between 6 months and 5 years. No significant effects were found for FT3 and FT3:FT4 ratio.

Conclusions: In euthyroid subjects, selenium supplementation minutely and dose-dependently affects thyroid function, when compared with placebo, by decreasing serum TSH and FT4 concentrations. Based on these findings, selenium supplementation is not warranted under conditions of marginal selenium deficiency. However, a role for selenium supplementation in the treatment of autoimmune thyroid diseases is still unresolved.

Publication types

  • Randomized Controlled Trial
  • Research Support, Non-U.S. Gov't

MeSH terms

  • Aged
  • Denmark
  • Dietary Supplements
  • Dose-Response Relationship, Drug
  • Double-Blind Method
  • Female
  • Humans
  • Male
  • Middle Aged
  • Selenium / administration & dosage
  • Selenium / blood
  • Selenium / pharmacology*
  • Thyroid Function Tests
  • Thyrotropin / blood
  • Thyrotropin / drug effects*
  • Thyroxine / blood
  • Thyroxine / drug effects*
  • Time Factors
  • Trace Elements / administration & dosage
  • Trace Elements / blood
  • Trace Elements / pharmacology*
  • Triiodothyronine / blood
  • Triiodothyronine / drug effects*
  • Yeast, Dried / administration & dosage


  • Trace Elements
  • Triiodothyronine
  • Thyrotropin
  • Selenium
  • Thyroxine