Context: There is concern whether soy phytoestrogens may affect thyroid function. If true, soy phytoestrogens may be expected to have a greater impact in subjects with subclinical hypothyroidism.
Objective: The primary aim was to determine the effect of soy phytoestrogen supplementation on thyroid function, with a secondary aim of assessing the effects on cardiovascular risk indices in patients with subclinical hypothyroidism.
Design and setting: We conducted a randomized, double-blind, crossover study in a tertiary care setting.
Participants: Sixty patients with subclinical hypothyroidism participated in the study.
Intervention: Patients were randomly assigned to either low-dose phytoestrogen (30 g soy protein with 2 mg phytoestrogens, representative of a Western diet) or high-dose phytoestrogen (30 g soy protein with 16 mg phytoestrogens, representative of a vegetarian diet) supplementation for 8 wk, then crossed over after an 8-wk washout period.
Main outcome measures: The primary outcome was progression to overt hypothyroidism, with secondary outcome measures of blood pressure, insulin resistance, lipids, and highly sensitive C-reactive protein (hsCRP).
Results: Six female patients in the study progressed into overt hypothyroidism with a standardized rate ratio of 3.6 (95% confidence interval, 1.9, 6.2) after 16-mg phytoestrogen supplementation. Both systolic and diastolic blood pressure decreased with 16 mg phytoestrogens, whereas systolic pressure alone decreased with 2 mg phytoestrogens. Insulin resistance (homeostasis model assessment of insulin resistance, 3.5 ± 0.09 vs. 2.6 ± 0.08; P < 0.02) and hsCRP (4.9 ± 0.04 vs. 3.9 ± 0.03; P < 0.01) decreased with 16 mg phytoestrogens. Lipid profile remained unchanged.
Conclusion: There is a 3-fold increased risk of developing overt hypothyroidism with dietary supplementation of 16 mg soy phytoestrogens with subclinical hypothyroidism. However, 16-mg soy phytoestrogen supplementation significantly reduces the insulin resistance, hsCRP, and blood pressure in these patients.