['A recent study on TSH receptor (TSHR) null mice suggested that skeletal loss occurring in hyperthyroidism is caused by the low TSH rather than high thyroid hormone levels. The aim of this study was to examine whether low TSH results in osteoporosis in the human.', 'We determined bone mineral density (BMD) and markers of bone metabolism in two male siblings aged 9.8 and 6.8 years with isolated TSH deficiency, due to a mutation of the TSH beta-subunit gene. BMD was measured in the lumbar spine (L1-L4) by dual-energy X-ray absorptiometry. Laboratory investigation included the determination of serum calcium, phosphate, 25-hydroxy-vitamin D, parathyroid hormone concentrations, and urine calcium (Ca)/creatinine (Cr) ratio. Osteoblast activity was measured by serum bone alkaline phosphatase and osteocalcin levels, and osteoclast activity by urine cross-linked amino-terminal, carboxy-terminal telopeptides of type I collagen and deoxypyridinoline concentrations.', 'BMD of both patients was within the normal range for age and sex; z-scores were -0.55 and -0.23 for patients 1 and 2 respectively. Serum calcium, phosphate, urine Ca/Cr ratio, and specific markers of bone metabolism were also within normal range.'] Are low TSH levels associated with osteoporosis in childhood?