Endocrine Abstracts

In developmental studies of mice lacking T3-receptor alpha (TRa^0/0) and beta (TRb^−/−) we demonstrated delayed endochondral ossification, reduced mineralisation and short stature in TRa^0/0 mice, despite euthyroidism. In contrast, TRb^−/− mice, which display thyroid hormone resistance with elevated T4 and T3 levels, have advanced ossification, increased mineralisation and accelerated growth. T3-target gene studies indicate that TRa^0/0 mice have skeletal hypothyroidism, whereas TRb^−/− bone is thyrotoxic. To investigate whether these abnormalities influence the adult skeleton, we studied euthyroid and T4-treated 22 week-old TRa^0/0 and TRb^−/− mice. There was no difference in bone length between WT, TRa^0/0 and TRa^0/0(+T4) mice, whereas TRb^−/− and TRb^−/−(+T4) bones were shorter by 5% and 4%, respectively (P<0.01). Histology revealed an 11% increase in cortical thickness in TRa^0/0 mice but 16% reduction in TRb^−/−. T4 treatment reduced cortical thickness by 27% in TRa^0/0 mice (P<0.05) and 29% in TRb^−/− (P<0.001). Bone micro-architecture and micro-mineralisation densities, analysed by quantitative backscattered electron scanning electron microscopy, demonstrated a 2.8-fold increase in trabecular bone volume (BVF) in TRa^0/0 mice (P<0.01) and increased trabecular thickness and complexity. Extension of trabecular bone into the diaphysis increased from 10% of total bone length in WT to 29% in TRa^0/0 (P<0.01). T4-treatment reduced trabecular thickness and complexity and BVF by 42% (P<0.05), and trabecular bone extent by 20% (P<0.05) in TRa^0/0 mice. In contrast, all trabecular bone parameters were reduced in TRb^−/− mice and unaffected by T4-treatment. Quantitative micro-mineralisation measurements were similar in TRa^0/0 and WT animals, whereas TRb^−/− mice had reduced trabecular and cortical mineral concentration (P<0.001). Thus, adult TRa^0/0 mice have increased bone of normal mineralisation density, whilst TRb^−/− mice are osteoporotic. These data demonstrate that TRalpha plays critical roles in skeletal development, and in the establishment and maintenance of adult bone structure. TRb can partially compensate for TRa in TRa^0/0 mice.