Triiodothyronine increases responsiveness of cultured rat bone cells to parathyroid hormone.

Osteoblast-like cells prepared from calvaria of newborn rats and grown in culture for 1 week show markedly increased ornithine decarboxylase (ODC) activity upon exposure to parathyroid hormone (PTH) for 4 h. Triiodothyronine (T3) increases ODC activity of the cultures in long-term experiments but does not stimulate cell replication. Moreover, PTH responsiveness is enhanced by T3. Thus, T3 acts directly on bone cells, and the clinical observation of bone sensitization to PTH by thyroid hormones is confirmed at the cellular level.