Background and Aim; It is controversal whether the long-term treatment with thyroid hormone given at supressive doses has a negative effect on bone metabolism. The aim of this study was to determine whether long-term thyroxine treatment was a risk factor for development of osteoporosis.
Patients and Methods: A selected group of women in premenopausal period (N=19) and a selected group of men (N=9) suffering from differentiated thyroid gland carcinoma in the average age of 39.0+/-8.0 years and 41.8+/-10.0 years were examined. In all of them, total thyreoidectomy was performed and thyroxine supressive therapy was introduced. The duration of supressive therapy from the beginning of this investigation was, for female examinees, 9.4+/-6.4 years and for males 8.1+/-6.0 years. The average dose of thyroxine was 2.5+/-0.5 mg per kg body weight for female examinees and 2.2+/-0.6 mg per kg body weight for male examinees. The prospective study of bone densitometry was done during one year on all examinees using the method of dual photon x-ray absorptiometry (DXA) of the spine and femoral neck and also by the method of one-photon absorptiometry (SPA) of the distal radius. Prior to bone densitometry, lab tests were done to all examinees. Thyroid gland hormones T3, T4 and TSH were determined, as well as the indicators of calcium metabolism in the samples of 2 and 24 hour urine (Ca and P) tests, and serum calcium and phosphorus and PTH. Subsequently, biochemical markers of bone formation (osteocalcin, alkaline phosphatase and PICP) and biochemical markers of bone resorption (Ca/Cr, and ICTP) were determined.
Results: Stastically significant loss of bone tissue was registered only on the distal radius concerning male examinees (p<0.05). On the lumbar part of the spine and femoral neck, only a minor bone loss was registered in the small number of examinees, and it was of no significant relevance either for male or female examinees. Values of T3, T4 and TSH were comparable to those in other studies. Serum calcium levels in one third of the female examinees and somewhat lower number of male examinees, turned towards the upper limit of the reference range or slightly over that limit, which correspondes with published data. The amount of PTH in serum of most examinees was below the medium value for this variable. The best biochemical marker of bone formation, especially considering female examinees, was osteocalcin, because almost 50% of female examinees had values over upper limit. Alkaline phosphatase and PICP were significantly less specific. In relation to Ca/Cr in urine, ICTP was a more sensitive biochemical marker of bone resorption. In more than one third of female and somewhat lower number of male examinees the values ranged above the upper limit. These two variables showed slight correlation with bone density.
Conclusions: It is necessary to prolonge the time of observation in prospective studies, as well as to increase the number of examinees in order to determine the relative risk of thyroxine supressive therapy on osteoporosis. Bone densitometry should be included in diagnostic procedure in order to discover persons with high risk for osteoporosis and to undertake a preventive treatment.