Exogenous triiodothyronine activates bone remodeling.

The effect of exogenous triiodothyronine (T3) as an activator of bone remodeling was investigated by measuring biochemical marker levels. Fourteen people received 100-160 micrograms of T3 for 1 week and were followed for a total of 10 weeks. Serum T3 increased markedly during the first week, whereas serum TSH and serum thyroxine (T4) decreased. Bone resorption was stimulated during the first week as evaluated from serum calcium and renal excretion of calcium, phosphate, and hydroxyproline. Serum osteocalcin showed an increase in the first week, probably caused by stimulation of existing osteoblasts. Serum alkaline phosphatase increased from week 6 to week 8, and serum osteocalcin increased at week 8 to a level higher than the initial level. The results indicate that T3 acts as an activator of bone remodeling and initiates bone resorption followed by bone formation. The observed timetable for the remodeling sequence is in accordance with that obtained by bone histomorphometry. Osteopenic states may be treated by synchronizing the skeletal remodeling processes and then selectively depressing bone resorption with calcitonin or diphosphonates (ADFR treatment). Synchronization may be achieved by short-term treatment with a drug that stimulates the formation rate of new remodeling cycles (an activator). The results from this study indicate that exogenous T3 may be used as an activator.