Change of maternal thyroid function in twin-twin transfusion syndrome

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.