Human chorionic gonadotropin (hCG) is a glycoprotein hormone produced by the placenta, which has a similar structure to thyroid-stimulating hormone (TSH).Due to this similarity, hCG could exhibit TSH-like activity and stimulate thyroid gland to produce thyroid hormones, particularly thyroxine (T4).This condition is often seen at end of the first trimester of pregnancy (weeks 10-12). In these cases, maternal serum TSH concentration is reduced and T4 level is elevated, causing overt hyperthyroidism that could lead to thyrotoxicosis if remain untreated. Pregnant women with hyperemesisgravidarum experience nausea and vomiting. These subjects seem to have elevated levels of serum hCG, particularly at the end of the first trimester when it reaches its peak, eventually leading to hyperthyroidism. There are some reports suggesting that hyperemesis gravidarum might be due to hyperthyroidism associated with elevated hCG in pregnant women at weeks10-12 of pregnancy.