IS SKELETAL RESPONSIVENESS TO THYROID-HORMONE ALTERED IN PRIMARY OSTEOPOROSIS OR FOLLOWING ESTROGEN REPLACEMENT THERAPY

Hyperthyroidism is characterized by increased bone turnover and resorp tive activity. Similar changes in remodeling are seen in osteoporosis. To study the pathogenetic role of thyroid hormone in osteoporosis, we measured concentrations of free and total thyroid hormones and invest igated the sensitivity of the skeleton toward thyroid hormones in 14 o steoporotic, 16 estrogen-treated, and 15 normal postmenopausal women w ith comparable thyroid status. Triiodothyronine T-3, 60 mu g/day for 7 days) was administered to the three groups. The skeletal response nas assessed by monitoring bone alkaline phosphatase (BAP), osteocalcin ( BGP), and pyridinium cross-linked telopeptide domain of type I collage n (ICTP) in serum and urinary excretion of hydroxyproline (OHP), pyrid inoline (PYR), and deoxypyridinoline (DPR) at days 0, 8, 15, and 57. W omen on estrogen replacement therapy exhibited lower bone turnover tha n the normal postmenopausal women. Markers of bone formation mere redu ced by 19-43% and markers of resorption by 22-48%. Tbe osteoporotic wo men displayed lower bone mass at the lumbar spine and the distal forea rm (p < 0.01-0.001), but the levels of biochemical markers of bone for mation and resorption vr ere comparable to values obtained in the norm al postmenopausal women. T-3 stimulation caused significant increases (p values ranging between 0.05-0.001) in all three groups of the resor ptive markers: ICTP (47%, 47%, 35%), OHP (29% 30%, 33%), PYR (43%, 27% , 51%), and DPR (42%, 24%, 59%). Of the formative markers, only BGP in creased significantly (32%, 40%, 47%) (p < 0.001). At day 57, however, all three formative markers increased compared with day 15 (p < 0.05- 0.001). No significant differences in bone markers were demonstrated b etween groups. In the osteoporotic group, as the only group, serum cal cium increased (p < 0.05) and serum PTH fell (p < 0.05). In conclusion , osteoporosis and estrogen substitution are not characterized by alte red concentrations of thyroid hormones or responsiveness to thyroid ho rmones at the level of individual bone cells; however, altered respons es pertaining to PTH and calcium were detected.