Increased rat femur osteocalcin mRNA concentrations following in vivo administration of thyroid hormone

Thyroid hormone has a direct resorptive effect on bone. Thyroid hormone therapy in doses that suppress pituitary TSH production result in a reduction in bone density. Osteocalcin is a bone matrix protein. Serum levels are a sensitive marker for bone turnover and are increased in hyperthyroid patients. In order to establish an animal model to study the effects of thyroid hormone on bone turnover, we measured rat femur osteocalcin mRNA following in vivo administration of thyroid hormone. Young CD rats weighing 60-90 g were given daily ip injections of T3, T4, or saline (control) for 12 days. Blood was obtained for radioimmunoassays, and RNA was extracted from femurs and analyzed by Northern blot using a 60-mer synthetic oligonucleotide probe corresponding to bases 360-420 of rat osteocalcin mRNA, labeled with [32P] ATP by 5'-end-labeling. Serum TSH concentrations were suppressed to subnormal levels by the lowest doses of T3 and T4, and to undetectable levels by the higher doses. Increases in serum T3 and T4 concentrations were proportional to the dose of each administered hormone. T3, 5 and 10 micrograms/100 g body weight, resulted in a 43% and 62% increase in osteocalcin mRNA, respectively. T4, 5, 10, and 20 micrograms/100 g body weight, resulted in a 35%, 47%, and 135% increase in osteocalcin mRNA, respectively. These data demonstrate that in vivo administration of either T4 or T3 to young rats results in a significant dose-dependent increase in femur osteocalcin mRNA concentrations.