The thyroid is an endocrine gland synthesizing, storaging and secreting thyroxine (T4) and triiodothyronine (T3). Currently, there are more and more reports and evidences that various chemical contaminants present in the environment, mainly polychlorinated biphenyls, interfere with stages of regulation, synthesis, secretion, transport of thyroid hormones. That can have a significant negative impact on the human body's endocrine homeostasis.