Context:Patients with pseudohypoparathyroidism type 1a (PHP-1a) develop early-onset obesity. The abnormality in energy expenditure and/or energy intake responsible for this weight gain is unknown.Objective:The aim of this study was to evaluate energy expenditure in children with PHP-1a compared with obese controls.Patients:We studied 6 obese females with PHP-1a and 17 obese female controls. Patients were recruited from a single academic center.Measurements:Resting energy expenditure (REE) and thermogenic effect of a high fat meal were measured using whole room indirect calorimetry. Body composition was assessed using whole body dual energy x-ray absorptiometry. Fasting glucose, insulin, and hemoglobin A1C were measured.Results:Children with PHP-1a had decreased REE compared with obese controls (P<0.01). After adjustment for fat-free mass, the PHP-1a group's REE was 346.4 kcals day(-1) less than obese controls (95% CI (-585.5--106.9), P<0.01). The thermogenic effect of food (TEF), expressed as percent increase in postprandial energy expenditure over REE, was lower in PHP-1a patients than obese controls, but did not reach statistical significance (absolute reduction of 5.9%, 95% CI (-12.2-0.3%), P=0.06).Conclusions:Our data indicate that children with PHP-1a have decreased REE compared with the obese controls, and that may contribute to the development of obesity in these children. These patients may also have abnormal diet-induced thermogenesis in response to a high-fat meal. Understanding the causes of obesity in PHP-1a may allow for targeted nutritional or pharmacologic treatments in the future.International Journal of Obesity advance online publication, 11 December 2012; doi:10.1038/ijo.2012.200.