Abstract
Background: The health benefits of soy are widely reported but there are queries on the effect of soy isoflavones on thyroid function and the underlying mechanism of action.Materials and Methods: We examined the effect of soy isoflavones on reverse tri-iodothyronine (or 3,3′,5′-tri-iodothyronine; rT3) in two studies comprising 400 patients: 200 men (study 1; 3 months) and 200 post-menopausal women (study 2; 6 months) who were randomized to consume 15 g soy protein with 66 mg of isoflavones (SPI) daily, or 15 g soy protein alone without isoflavones (SP) daily.Results: SPI supplementation increased rT3 serum concentration in both men 0.41 (0.12) vs. 0.45 (0.14) nmol/L and women 0.33 (0.12) vs. 0.37 (0.09) nmol/L at 3 months compared to SP that was not seen at 6 months. Thyroid stimulating hormone (TSH) serum concentrations increased while free thyroxine (fT4) concentrations decreased with 3 months of SPI compared to SP supplementation for both men and women. rT3 correlated with TSH in both studies (p = 0.03) but not with either fT3 or fT4. fT3 levels did not differ between the SPI and SP preparations.Conclusion: Soy isoflavones transiently increased rT3 levels within 3 months though reverted to baseline at 6 months. The mechanism for this would be either rT3 degrading deiodinase 1 and/or deiodinase 2 activities are transiently inhibited at 3 months, or inhibition of deiodinase 3, which generates rT3 from T4 is induced at 6 months. These changes were mirrored in the TSH concentrations, suggesting that short-term high dose isoflavone transiently impairs thyroid function in the first 3 months and may impact on general health during this period.ISRCTN Registry: ISRCTN 90604927; ISRCTN34051237.
Highlights
The consumption of soy food products have increased due to the reported potential health benefits that have been suggested to be due to the isoflavone components, leading to the development of isoflavone supplements and the fortification of foods with isoflavones [1, 2]
In Study 1, there was a significant increase in rT3 with SPI supplementation [0.41 (0.12) vs. 0.45 (0.14) nmol/L] at 3 months compared to SP supplementation [0.43 (0.10) vs. 0.40 (0.15) p-value < 0.001] (Tables 1, 2) in men with type 2 diabetes
There was a significant correlation between changes in rT3 with TSH (r = 0.52; p = 0.03) but no correlation with changes in fT3 (r = 18; p = 0.81) and fT4 (r = 13; p = 0.62 0.81)
Summary
The consumption of soy food products have increased due to the reported potential health benefits that have been suggested to be due to the isoflavone components, leading to the development of isoflavone supplements and the fortification of foods with isoflavones [1, 2]. There are concerns in susceptible individuals that soy may adversely affect thyroid function [10,11,12,13,14]. The mechanism by which soy isoflavones may interfere with thyroid function is unclear, but it is critical to understand given the wide spread use of soy products. Animal studies have suggested that soy isoflavones interfere with thyroid function via thyroid peroxidase inhibition, as well as, with tissue deiodinase enzyme activities, which may affect extrathyroidal thyroid hormone metabolism, including rT3 concentrations [15]. The health benefits of soy are widely reported but there are queries on the effect of soy isoflavones on thyroid function and the underlying mechanism of action
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