Thyroid Research Thyroid Research Archive Drug Effects

Kelp has a weak antithyroid effect in normal subjects

(March 2004)

The background of the study. High doses of iodine have a weak antithyroid effect in normal subjects, and can cause hypothyroidism or hyperthyroidism in patients with thyroid disease. Kelp is a type of seaweed that contains large amounts of iodine. It is eaten as food or in capsule form as a nutritional supplement. Given its iodine content, it has the potential to cause thyroid dysfunction. In this study the effects of kelp on thyroid function were determined in normal subjects.

How the study was done. Thirty-six normal subjects were randomly assigned to take four capsules of kelp (high-dose kelp group), two capsules of alfalfa and two capsules of kelp (low-dose kelp group), and four capsules of alfalfa (control group) daily for four weeks. The iodine content of the kelp capsules was 330 µg. Serum thyrotropin (TSH) and free thyroxine (T4) were measured at base line and four weeks, and two weeks after cessation of kelp. At base line and four weeks, serum TSH was measured 30 minutes after the intravenous injection of thyrotropin-releasing hormone (TRH).

The results of the study. In the two kelp groups, serum TSH concentrations increased slightly, but serum free T4 concentrations did not change. The serum TSH response to TRH increased only in the high-dose kelp group. Urinary iodide excretion increased substantially in both kelp groups. Two weeks after kelp ingestion was stopped, serum TSH and free T4 concentrations were similar to those at base line.

The conclusions of the study. Ingestion of kelp for four weeks has a weak antithyroid action in normal subjects.

The original article. Clark CD, Bassett B, Burge MR. Effects of kelp supplementation on thyroid function in euthyroid subjects. Endocr Pract 2003;9:363-9.