HUMAN CHORIONIC GONADOTROPIN (hCG) is part of the family of glycoprotein hormones, which have a common alpha-subunit and a unique beta-subunit. This family includes thyroid stimulating hormone (TSH). As a result, hCG has weak thyroid-stimulating activity [1]. In normal pregnancy, serum hCG concentrations rise soon after implantation and peak at 10 to 12 weeks of gestation, after which they decline. During the period of highest serum hCG concentrations, serum free triiodothyronine (fT3) and free thyroxine (fT4) concentrations increase, usually within the normal range, and serum TSH concentrations are correspondingly reduced. Transient subclinical hyperthyroidism occurs in 10% to 20% of normal pregnant women during the period of highest serum hCG concentrations; these women do not require treatment [2].Change of maternal thyroid function in twin-twin transfusion syndrome Masachi Hanaoka 1), 2) , Naoko Arata 2) and Haruhiko Sago Abstract. Human chorionic gonadotropin (hCG) has weak thyroid-stimulating activity because of its homology with thyroid stimulating hormone (TSH). In twin-twin transfusion syndrome (TTTS), which is a severe complication of monochorionic twin pregnancies, a close association between maternal serum hCG concentration and TTTS has been reported. And, TTTS can be treated by fetoscopic laser coagulation of the communicating vessels. To clarify the relationship between maternal serum hCG and maternal thyroid function in TTTS, the present study investigated the change in thyroid hormone and hCG levels after laser therapy. The protocol included collection of serial maternal blood samples in TTTS before laser therapy, and at two and four weeks after laser therapy. For 131 cases of TTTS, the following parameters were determined at each point: hCG, TSH, free triiodothyronine (fT3), and free thyroxine (fT4). The multiple of the median (MoM) of pre-operative hCG concentration in TTTS was 5.39 MoM (interquartile range, 2.83 -8.64). There was a moderate positive correlation between hCG and fT3 in TTTS pre-operatively (R = 0.22, P = 0.030). fT4 was also positively correlated with hCG (R = 0.33, P < 0.001). Some cases showed very high concentration in fT3. When laser therapy for TTTS was effective, the hCG concentration significantly decreased, and fT3 and fT4 decreased progressively in concert with the decrease in hCG. The relationship between hCG and thyroid function in TTTS supports the finding of TTTS as a novel etiology of hCG-mediated hyperthyroidism during pregnancy.Key words: Pregnancy, Human chorionic gonadotropin (hCG), Twin-twin transfusion syndrome (TTTS), Hyperthyroidism, Laser thearpyTwin-twin transfusion syndrome (TTTS) accounts for approximately 10% of monochorionic twin pregnancies. If left untreated, TTTS is associated with a high fetal morbidity and mortality rate [3]. A net transfusion of blood flow from one fetus (donor twin) to the other (recipient twin) via placental vascular anastomoses has been proposed as the major etiology of TTTS [4,5]. The donor twin becomes hypovolemic and oli...