Otun, J., Sahebkar, A., Östlundh, L., Atkin, S. L., & Sathyapalan, T. (2019). Systematic Review and Meta-analysis on the Effect of Soy on Thyroid Function. Scientific reports. https://doi.org/10.1038/s41598-019-40647-x
Otun J, Sahebkar A, Östlundh L, Atkin SL, Sathyapalan T. Systematic Review and Meta-analysis on the Effect of Soy on Thyroid Function. Scientific reports. 2019. doi:10.1038/s41598-019-40647-x
Otun, J., et al. "Systematic Review and Meta-analysis on the Effect of Soy on Thyroid Function." Scientific reports, 2019.
Soy supplements may raise TSH, with no significant T3 or T4 change
A meta-analysis of 18 randomized trials in adults found that soy supplements produced no significant change in free T3 or free T4 levels. Thyroid-stimulating hormone (TSH) rose slightly (p = 0.049). The authors consider the clinical meaning of that small TSH rise unclear.
Key Findings
The review pooled 18 randomized controlled trials in adults. Of these, 21 and 22 treatment arms contributed to the free T3 and free T4 analyses, and 26 to the TSH analysis.
Free T3 and free T4 showed no significant change with soy supplementation (free T3 p = 0.499, free T4 p = 0.656).
TSH rose significantly with soy supplementation (p = 0.049). The lower edge of the 95% confidence interval sat barely above zero, and the result was sensitive to one treatment arm.
The TSH rise reached significance in trials lasting three months or longer (p = 0.028) and in people with subclinical hypothyroidism at baseline (p < 0.001).
Two trials enrolled men and women, three enrolled men only, and the rest enrolled women only. Two trials enrolled people with subclinical hypothyroidism.
Interpretation
The study is a systematic review and meta-analysis. It pools randomized controlled trials found through a structured search. The trials tested mainly supplements of soy isoflavones, soy extract or soy protein. The authors note that some trials were small and many were short. They recommend further research in people with subclinical hypothyroidism, a mild form of underactive thyroid.
RRM Context
Thyroid hormones affect ovulation and cycle length. Restorative Reproductive Medicine looks for the root cause of cycle and fertility problems in both partners. Diet is part of that picture.
Our editorial summary of this paper, not the article's abstract.
Abstract
Soy foods have had an important dietary role in Asian countries for centuries, and in recent years they have become increasingly popular in Western countries as a result of their suggested health benefits. Nevertheless, there are some concerns that soy can have a negative effect on thyroid function and can alter the levels of thyroid hormones. The aim of this systematic review was to investigate the link between soy or soy product consumption and thyroid function via the measurement of thyroid hormone levels. A systematic review and meta-analysis was undertaken on all randomised controlled trials of studies including soy as an intervention and where free triiodothyronine (fT3), free thyroxine (fT4) and thyroid stimulating hormone (TSH) was measured. The search included PubMed, MEDLINE, EMBASE, Cochrane and sources for the grey literature. Quantitative data synthesis was performed using a random-effects model, with standardized mean difference (SMD) and 95% confidence interval as summary statistics. A total of 18 articles were suitable for review. The meta-analysis showed no significant changes in fT3 (WMD: 0.027 pmol/L, 95% CI: -0.052, 0.107, p = 0.499; I2: 55.58%), fT4 (WMD: -0.003 pmol/L, 95% CI: -0.018, 0.011, p = 0.656; I2: 87.58%) while an elevation in TSH levels was observed (WMD: 0.248 mIU/L, 95% CI: 0.001, 0.494, p = 0.049; I2: 80.31%) levels with soy supplementation. There was no evidence of publication bias. Soy supplementation has no effect on the thyroid hormones and only very modestly raises TSH levels, the clinical significance, if any, of the rise in TSH is unclear.
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