Context: Soy phytoestrogens are suggested to have an adverse effect on thyroid function but the contribution to this by soy protein alone when free from isoflavones is unknown.
Objective: The primary aim was to determine the effect of isoflavone free soy protein supplementation on thyroid function, with a secondary aim of assessing the effects on cardiovascular risk indices in patients with subclinical hypothyroidism.
Design and setting: This was a randomised, double-blind, crossover study in a tertiary care setting.
Participants: Eighty patients with subclinical (compensated) hypothyroidism participated in the study.
Intervention: Patients were randomly assigned to either isolated soy (isoflavone free) protein (SP) or casein protein (CP) supplementation for 8 weeks, washed out for 8 weeks and then crossed over for a further 8 week period.
Main outcome measures: The primary outcome was a change in serum free thyroxine, with secondary outcome measures of progression to overt hypothyroidism, blood pressure, insulin resistance, fasting lipids, and high sensitivity C-reactive protein (hsCRP).
Results: Mean percentage change from baseline showed a significant decrease in free thyroxine (−18.1% vs +4.3%, P<0.01) with SP, but no patient developed overt hypothyroidism. There were significant decreases in fasting glucose (−22.7% vs +2.3%, P<0.01), insulin resistance (−3.4% vs +25.0%, P=0.02), total cholesterol (−13.7% vs +2.5%, P<0.01), triglycerides (−64.6% vs +14.1%, P<0.01) and hsCRP (−71.5% vs +23.6%, P<0.01) in the SP group compared to the CP group. Blood pressure, LDL and HDL remained unchanged in both groups.
Conclusion: Isoflavone free soy protein decreased serum free thyroxine in these patients with subclinical hypothyroidism though the decrease was likely not clinically significant. However, it did reduce the cardiovascular risk indices of fasting glucose, insulin resistance, total cholesterol, triglycerides and hsCRP compared to casein protein.