Childhood hypothyroidism delays ossification and bone mineralization, whereas adult thyrotoxicosis causes osteoporosis. To determine how effects of thyroid hormone (T3) during development manifest in adult bone, we characterized TRalpha1(+/m)beta(+/-) mice, which express a mutant T3 receptor (TR) alpha1 with dominant-negative properties due to reduced ligand-binding affinity. Remarkably, adult TRalpha1(+/m)beta(+/-) mice had osteosclerosis with increased bone mineralization even though juveniles had delayed ossification. This phenotype was partially normalized by transient T3 treatment of juveniles and fully reversed in compound TRalpha1(+/m)beta(-/-) mutant mice due to 10-fold elevated hormone levels that allow the mutant TRalpha1 to bind T3. By contrast, deletion of TRbeta in TRalpha1(+/+)beta(-/ -) mice, which causes a 3-fold increase of hormone levels, led to osteoporosis in adults but advanced ossification in juveniles. T3-target gene analysis revealed skeletal hypothyroidism in TRalpha1(m/+)beta(+/-) mice, thyrotoxicosis in TRalpha1(+/+)beta(-/-) mice, and euthyroidism in TRalpha1(+/)beta(-/-) double mutants. Thus, TRalpha1 regulates both skeletal development and adult bone maintenance, with euthyroid status during development being essential to establish normal adult bone structure and mineralization.