Effects of Kelp Supplementation on Thyroid Function in Euthyroid Subjects

Endocr Pract. Sep-Oct 2003;9(5):363-9. doi: 10.4158/EP.9.5.363.

Abstract

Objective: To study the effects of ingestion of two different doses of supplemental kelp on the thyroid function of healthy euthyroid subjects.

Methods: We conducted a double-blind prospective clinical trial involving 36 healthy euthyroid subjects, who were randomly assigned to receive placebo (4 alfalfa capsules per day), low-dose kelp (2 kelp capsules and 2 alfalfa capsules per day), or high-dose kelp (4 kelp capsules per day) for 4 weeks. Thyrotropin (thyroid-stimulating hormone or TSH), free thyroxine, and total triiodothyronine were assessed at weeks 0, 4, and 6. Response to thyrotropin-releasing hormone stimulation, urinary iodine excretion, and basal metabolic rate were determined at weeks 0 and 4.

Results: TSH concentrations did not differ significantly between week 0 and week 4 in the placebo group (P = 0.16) but increased significantly in both the low-dose kelp (P = 0.04) and high-dose kelp (P = 0.002) groups. Free thyroxine concentrations decreased slightly but significantly after 4 weeks of placebo but were unchanged in the low-dose and the high-dose kelp groups. In contrast, total triiodothyronine levels did not differ significantly after 4 weeks of placebo or low-dose kelp therapy but were significantly decreased after high-dose kelp therapy (P = 0.04). Similarly, the thyrotropin-releasing hormone stimulation test showed no significant change in poststimulation TSH after 4 weeks in the placebo or low-dose kelp groups but revealed a significantly increased response after high-dose kelp therapy (P = 0.0002). The 24-hour urinary iodine excretion showed dose-dependent increases in the two kelp study groups. Basal metabolic rate did not change significantly in any study group during the 4-week study period. All thyroid laboratory values returned to baseline 2 weeks after cessation of kelp supplementation, except for TSH in the high-dose kelp group, which was significantly decreased.

Conclusion: Short-term dietary supplementation with kelp significantly increases both basal and poststimulation TSH. These findings corroborate previous studies on the effects of supplemental iodide given to euthyroid subjects for a similar period. Further studies are needed to determine whether long-term kelp supplementation would cause clinically significant thyroid disease.

Publication types

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

MeSH terms

  • Adult
  • Basal Metabolism
  • Dietary Supplements*
  • Female
  • Humans
  • Iodine / urine
  • Kelp*
  • Male
  • Middle Aged
  • Placebos
  • Prospective Studies
  • Thyroid Gland / physiology*
  • Thyrotropin / blood
  • Thyrotropin-Releasing Hormone
  • Thyroxine / blood
  • Triiodothyronine / blood

Substances

  • Placebos
  • Triiodothyronine
  • Thyrotropin-Releasing Hormone
  • Thyrotropin
  • Iodine
  • Thyroxine