HCG mediated hyperthyroidism

hCG is a hormone the goes to high levels during pregnancy and is a thyroid stimulator that can cause three forms of hyperthyroidism during a normal pregnancy. 

Transient subclinical hyperthyroidism 

This condition is common and occurs in 10 to 20 percent of normal pregnant women during the first trimester which is the period of highest serum hCG concentrations. Levels of thyroid hormones should be checked every 4-6 weeks and most of these women do not require treatment.

Hyperemesis gravidarum

This is a syndrome defined as nausea and vomiting (severe morning sickness) associated with a weight loss of more than five percent during early pregnancy. Some degree of nausea with or without vomiting occurs in the majority of all pregnancies. Symptoms usually begin at 5 to 6 weeks of gestation, peak at 9 weeks, and usually disappear by 16 to 18 weeks of gestation. Fifteen percent of women have symptoms that may continue until the third trimester and until delivery in 5 percent. Many women  with hyperemesis have either sub-clinical or mild overt hyperthyroidism. This condition does not often require antithyroid treatment. 

Trophoblastic hyperthyroidism 

This condition occurs in about 60 percent of women with very abnormal ovaries (hydatidiform mole or choriocarcinoma). The hyperthyroidism may be severe, and is primarily treated by evacuation of the mole or therapy directed against the choriocarcinoma.