HCG and TSH
have identical α subunit, which may lead to cross reactivity
between the increased level of hCG and TSH receptor,
leading to stimulation of T3 and T4, in return causing
negative pituitary feedback of TSH, resulting in high T3
and T4 level, but low TSH, especially in 8th–14th gestational
week, when hCG level peaks during pregnancy.