Objective: Soy isoflavones are commonly used to alleviate menopause-related symptoms. Postmenopausal women are at increased risk for hypothyroidism, and there are concerns that isoflavones may be detrimental to thyroid health. The aim of this study was to examine the effects of soy protein and isoflavones on thyroid function and the relationship between thyroid function and ovarian function.
Methods: Adult female cynomolgus monkeys (Macaca fascicularis) were randomized to consume two diets differing only in protein source: casein-lactalbumin (n = 44) or soy protein with isoflavones (n = 41). After 34 months, all animals were ovariectomized via laparotomy. Half of the monkeys from each diet treatment group continued to consume their preovariectomy treatment phase diet (either isolated soy protein [n = 19] or casein-lactalbumin [n = 21]) for an additional 34 months. The remaining animals did not continue their diets and thus were not considered further. Circulating progesterone, triiodothyronine, thyroxine, and thyroid-stimulating hormone were measured at baseline. Thyroid hormones were remeasured during each treatment phase.
Results: Dietary soy increased triiodothyronine in preovariectomized monkeys and prevented a decline in thyroxine after surgical menopause (both P’s < 0.05). Mean progesterone concentrations were positively correlated with triiodothyronine at baseline in preovariectomized monkeys (P < 0.01).
Conclusions: Progesterone levels and triiodothyronine are positively correlated in macaques. Dietary soy increases triiodothyronine in preovariectomized monkeys and prevents a decline in thyroxine after surgical menopause. The outcomes observed in this study suggest that soy protein and isoflavone consumption does not adversely affect—and may even preserve—thyroid function in postmenopausal women.
From the 1Integrative Physiology and Pharmacology Graduate Program, Section on Comparative Medicine, Department of Pathology, Wake Forest University School of Medicine, Winston-Salem, NC; and 2Section on Comparative Medicine, Department of Pathology, Wake Forest University School of Medicine, Winston-Salem, NC.
Received August 30, 2013; revised and accepted January 21, 2014.
Funding/support: This work was supported, in part, by the National Center for Research Resources (grant S10 RR020890), the National Heart, Lung, and Blood Institute (grants P01 HL45666 and R01HL79421 to J.R.K. and grant 2 R01 HL 087103 to C.A.S.), and the National Institutes of Health (grant T32 OD 010957 to M.G.S.).
Financial disclosure/conflicts of interest: None reported.
Address correspondence to: Carol A. Shively, PhD, Section on Comparative Medicine, Department of Pathology, Wake Forest University School of Medicine, Medical Center Boulevard, Winston-Salem, NC 27157-1040. E-mail: firstname.lastname@example.org