Effects of kelp supplementation on thyroid function in euthyroid subjects

Kelp has numerous health benefits, including thyroid, hair, and skin health.

A double-blind prospective clinical trial was conducted involving 36 healthy euthyroid (thyroid gland is functioning normally) subjects. Subjects 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 (T3) 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 (T3) 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. 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 thyroid-stimulating hormone (TSH). These findings corroborate previous studies on the effects of supplemental iodide given to euthyroid subjects for a similar period.

Clark CD et al. Effects of kelp supplementation on thyroid function in euthyroid subjects. Endocr Pract. Sep-Oct 2003; 9(5): 363-9.