Endocrine Abstracts

: In patients with subclinical hypothyroidism, soy protein and isoflavone combination has been shown to increase the risk of developing overt hypothyroidism; however, it is unclear if soy affects thyroid function in those without existing thyroid compromise.

Materials and methods: Two double blind randomised trials were undertaken in which thyroid function was performed routinely at the beginning and end as a secondary end point. In both trials soy protein 30g with and without 66mg of soy isoflavones were used (preparation 1); soy protein alone that was isoflavone free (less than 300 parts per billion following serial ethanol washing)(preparation 2) was used for a period of 12 weeks. Study 1: 200 women within two years of the onset of the menopause to determine the safety of soy on markers of bone turnover as the primary end point. Study 2: 200 men with T2DM with a total testosterone level ≤12 nmol/l with normal gonadotrophins with the primary end point of testosterone change. Here the secondary thyroid endpoints are presented.

Results: All patients had normal thyroid function at baseline in both the studies. In post menopausal women there was a significant increase in TSH (1.58±0.93 vs 2.57±1.19 mU/l; P value <0.01) and reduction of free thyroxine (1.08±0.17 vs 0.92±0.17 ng/dl; P value 0.02) after preparation 1 after 3 months compared to baseline. In men with type 2 diabetes there was a significant increase in TSH (1.82±0.10 vs 3.28±0.11 mU/l; P value <0.01) and reduction in fT₄ (0.98±0.02 vs 0.86±0.02 ng/dl; P value <0.01) with preparation 1 but not after preparation 2 after 3 months compared to baseline. None of the patients developed subclinical or overt hypothyroidism during this period. There were no changes in fT₃ after either preparation 1 or preparation 2. There was no significant change in TSH or thyroxine after supplementation of preparation 2.

Conclusions: There was a significant increase in TSH and reduction in free T₄ only after soy protein with isoflavones suggesting a direct effect on the thyroid that is unlikely to be clinically significant unless thyroid compromise is already present. We have previously shown that isoflavone alone has no effect on thyroid function suggesting that the isoflavones in the soy protein matrix is active component.