There has been much discussion of late over the possible adverse effect of soy consumption on thyroid function in both infants and adults. Recently, several studies have provided insight into the relationship between soy and thyroid function. Many researchers indicates that the consumption of isoflavone – rich soy protein had little effect on thyroid hormone level. So, it is important to study the effect of soybean on thyroid hormones. Three formulas of soybean, i.e., whole meal, low fat flour and dehulled ones, were used in this study and chemically analyzed. Female rats were fed on soybean (whole meal, low fat flour and dehulled) for six weeks with or without exo-thyroid hormone. Biological evaluation of the experimental rats such as: body weight gain, weight of some organs, thyroid hormones, (T₃, T₄), TSH, prolactin and progesteron hormones were estimated. Data showed that feeding soybean resulted in loss weight of animals and weight of organs and affected TSH, T₃ secretion followed by T₄. Also, it increased progesterone hormone, but no effect on prolactin hormone.