A 15-year-old boy complains of periodical headaches and fatigability. On examination, the boy is active, his psychoemotional development corresponds with his age, his skin is pale and clammy. No abnormalities of the internal organs were detected. Blood pressure is 120/80 mm Hg. Noticeable is the excessive subcutaneous fat that is evenly distributed throughout the body. The doctor suspects obesity in the child. What parameter should be measured first to confirm his diagnosis?