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.