PFOA exerts hormonal effects including alteration of thyroid hormone levels. Testosterone is the primary male sex hormone and anabolic steroid. Levonorgestrel is used in combination with an estrogen in menopausal hormone therapy. Phytoestrogens are chemicals which act like the hormone estrogen. Progesterone is released in response to high blood levels of luteinizing hormone. It has receptors for estrogen, progesterone, growth hormone, and others. They produce testosterone in the presence of luteinizing hormone (LH). Pregnancy leads to marked changes in thyroid hormone physiology. It is a progestin and has effects similar to those of the hormone progesterone. Examples include thyroid hormone, growth hormone, and insulin. The structure of prolactin is similar to that of growth hormone and placental lactogen. Insulin was the first peptide hormone discovered. Treatment consists of a daily dose of thyroid hormone (thyroxine) by mouth. Most people are treated with levothyroxine, or a similar synthetic thyroid hormone. They also produce peptides involved in ovarian hormone synthesis regulation.