Natural Solutions Radio header image

The Effect of Soy Phytoestrogen Supplementation on Thyroid Status and Cardiovascular Risk Markers


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.

Thozhukat Sathyapalan, Alireza M. Manuchehri, Natalie J. Thatcher, Alan S. Rigby, Tom Chapman, Eric S. Kilpatrick and Stephen L. Atkin
Department of Academic Endocrinology, Diabetes and Metabolism (T.S., A.M.M., S.L.A.), Hull York Medical School, Hull HU3 2JZ, United Kingdom; Food Standards Agency (N.J.T.), London WC2B 6NH, United Kingdom; Department of Academic Cardiology (A.S.R.), University of Hull, Hull HU3 2JZ, United Kingdom; Essential Nutrition Ltd. (T.C.), Brough HU15 1EF, United Kingdom; and Department of Clinical Biochemistry (E.S.K.), Hull Royal Infirmary, Hull, HU3 2JZ United Kingdom

Address all correspondence and requests for reprints to: T. Sathyapalan, Michael White Diabetes Centre, Hull Royal Infirmary, 220-236 Anlaby Road, Hull HU3 2JZ, United Kingdom. E-mail: e-mail address is being protected from spambots, you need JavaScript enabled to view it .

The Journal of Clinical Endocrinology & Metabolism, doi:10.1210/jc.2010-2255
J Clin Endocrinol Metab, Vol. 96 (5): 1442-1449
Copyright © 2011 by The Endocrine Society