Introduction
The soy plant and its products have an important place in the nutrition profiles of many countries, especially of Asian countries. The preventive effects of soy have been demonstrated against heart diseases [1-2] breast cancer, prostate cancer [3-4] and osteoporosis[5]. Despite those benefits, soy acts on the thyroid gland unfavourably. Several studies are available in the literature, reporting cases with hypothyroidism, goitre, and increased iodine need in association with the consumption of soybean.
Isoflavones constitute the most commonly studied component of soy. Soybean and most of its highly consumed products in Western populations contain isoflavones in glycosidic forms as genistin, daidzin, and glycitin. After oral intake, isoflavone glycosides are hydrolyzed by the intestinal microflora and converted into bioactive aglycone forms [6]. The aglycones genistein and daidzein irreversibly inactivate thyroid peroxidase; which is the enzyme binding iodine to the thyroglobulin molecule [7]. As is known, the production of T3 and T4 hormones requires iodine to be incorporated into the thyroglobulin molecule in the thyroid tissue [8-10]. These activities of genistein and daidzein increase the iodine demand in the organism and induce hypothyroidism [7]. In vitro studies demonstrate that genistein and daidzein show their effects even at very low concentrations [11].
Despite their known unfavourable effects on thyroid functions, it is noteworthy that soy and its products are still used extensively in infant formulas. Although studies about this subject matter are available in the literature, no studies have compared different doses of soy or investigated the histopathological effects of soy on the thyroid gland. We evaluated the effects of infant formulas with different quantities of soy content on the functional and histopathological characteristics of the thyroid gland.