Methods
The study protocol was approved by the local Animals Ethics Committee. Twenty-eight female Sprague-Dawley rats (one-month-old, mean weight 55g, weight range: 3-85g) were divided into four equal groups to carry out statistical analyses in a 95% confidence interval and at a sample power of 0.9. The subjects were kept at standart fabricated cages of 40x25x25 cm in size with a stainless steel roof, plastic walls, and a plastic floor covered with wood shavings. The cages were exposed to 12 hours light and 12 hours dark cycles in a day. All procedures were carried out in compliance with the guidelines for the care and use of laboratory animals in the Declaration of Helsinki. The groups in the experiment and the diets given to the subjects in each groups are listed below:
Group 1 (n=7): Standard pellet rat food (8g/day)
Group 2 (n=7): Soy-free formula (8g/day, Hipp®)
Group-3 (n=7): Low-dose (1.12g/100ml) soy-containing formula (8 g/day, Similac®)
Group 4 (n=7): High-dose (2.64g/100ml) soy-containing formula (8g/day, Modilac®).
Before starting the feeding regimens, blood samples were collected from the tail veins of the rats on day 0 and days 30, 60, and 90 to evaluate the levels of free T3, free T4, thyroid stimulating hormone (TSH) , antithyroglobulin antibody (Anti TG), and anti-troponin antibody (Anti TPO). All subjects were weighed weekly and the measured body weights were recorded. All subjects were sacrificed by cervical dislocation on the 90th day of the experiment. After making an anterior longitudinal incision in the necks of the rats; their thyroid glands were totally excised, placed in containers containing formaldehyde, and submitted to the pathology laboratory for the histopathological examination. The tissue sections obtained from the thyroid glands of the rats were stained with hematoxylin and eosin. A pathologist blinded to the experimental groups examined the specimens and scored the histopathological findings according to the histopathological fibrosis scoring system (Table 1).