Hyperthyroidism is due to stimulation of the thyroid by hCG. The hCG is a glycoprotein hormone produced by trophoblastic tissue and composed of a unique β-subunit and an α-subunit that is similar to that of pituitary glycoprotein hormones, including TSH. Weak thyrotropic activity is seen with hCG≈0.04 to 0.5 µIU bovine TSH/IU hCG or 0.52±0.35 µIU human TSH/IU hCG.3,4 On a molar basis, hCG is 1/4000 as potent as human TSH in the TSH bioassay.5 Unlike normal pregnancies with β-hCG levels <100000 IU/L, in molar pregnancies values are often >200000. As the hCG level increases, its thyrotropic activity becomes more notable and the thyroid sufficiently stimulated to become hyperfunctioning. However, there is no direct correlation between the β-hCG and T4 or T3 levels (possibly because only a fragment of β-hCG and not the total β-hCG is measured on most assays). Therefore, the degree of elevation of the thyroid hormones cannot be predicted by the β-hCG level. The pituitary will decrease TSH production in response to the elevated T4 and T3 values, and the TSH level may be undetectable, as in Graves disease.