<i>Thra</i> knockout protects male mice from hyperthyroidism-driven cortical bone loss by mitigating bone resorption

Abstract Secondary osteoporosis is a major global health problem and can be caused by endocrine diseases, including hyperthyroidism. Of note, hyperthyroid mice exhibit both trabecular and cortical bone loss, accompanied by reduced bone strength, as a consequence of a high bone turnover in which bone resorption exceeds bone formation. Recently, we discovered that osteocytes in bones obtained from hyperthyroid mice develop osteoclast-like features. However, the role of thyroid hormone receptor α (TRα) in hyperthyroidism-driven bone resorption remains ill-defined. Here, we rendered male Thra0/0 mice (which lack all TR□ isoforms) hyperthyroid, and comprehensively characterized their bone phenotype. Thra0/0 mice with L-thyroxine (T4) treatment displayed trabecular, but not cortical bone loss, and maintained bone strength, while T4-treated wildtype littermates (WT) showed overall reduced bone mass and strength. Serum levels of bone resorption marker TRAP and numbers of TRAP-positive osteoclasts were elevated in hyperthyroid WT, but not Thra0/0 mice. We also detected increased numbers of TRAP-positive osteocytes in long bones of T4-treated WT mice. In line, the expression of osteoclast marker genes was upregulated with T4 in bone tissue from WT mice only. While thyroid hormones have been reported not to directly affect osteoclast activity, our in vitro studies using late osteoblasts/osteocytes derived from Thra0/0 and WT mice revealed that the thyroid hormone-induced expression of osteoclast markers in these cells was Thra-dependent. Further, the Rankl/Opg ratio linking osteocytes to enhanced osteoclastogenesis was upregulated in hyperthyroid WT, but not Thra0/0 cells. Taken together, we demonstrate that Thra knockout can preserve cortical bone mass and bone strength by obstructing an increase of bone-resorbing osteoclasts and osteocytes with osteoclast-like features in hyperthyroid male mice.