Effects of Kelp Supplementation on Thyroid Function in Euthyroid Subjects

Abstract

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