Monitoring L-thyroxine therapy: lessons from the effects of L-thyroxine on bone marrow density

Article Abstract:

Treatment with the thyroid hormone thyroxine is used to replace deficient levels of this hormone in patients with hypothyroidism (reduced thyroid function) or to suppress the growth of abnormal thyroid tissue. However, L-thyroxine therapy can cause bone loss. Thyroid hormone treatment decreases bone density by enhancing bone resorption, a process in which the bone is dissolved and absorbed into tissue. A recent study suggested that bone loss may be reduced by carefully monitoring thyroxine treatment. This is accomplished by measuring the levels of a pituitary hormone, thyroid-stimulating hormone (TSH), which activates the release of thyroxine from the thyroid gland. Increased levels of thyroxine are normally associated with reduced levels of TSH. Hence, in patients with hypothyroidism or reduced levels of thyroxine, replacement therapy with L-thyroxine should increase blood levels of thyroxine and reduce TSH levels to normal values. Patients who are taking L-thyroxine to suppress abnormal thyroid growth require more than the replacement doses. Such patients do not develop symptoms from large L-thyroxine doses, but are considered to have subclinical hyperthyroidism, a condition characterized by elevated blood levels of thyroxine. Hence, in patients requiring suppressive L-thyroxine treatment, TSH levels are usually reduced to subnormal, or below normal values. Several studies have shown that subclinical hyperthyroidism resulting from excessive doses of L-thyroxine is associated with bone loss, particularly in women who have not stopped menstruating. Although careful monitoring of L-thyroxine treatment helps to reduce bone loss, there will always be some degree of bone loss, which in the long-term may contribute to the occurrence of hip fracture. Hence, the need for L-thyroxine treatment must be carefully considered, and the dose of L-thyroxine necessary to reduce the TSH level should be related to the clinical condition. For example, large doses of L-thyroxine are appropriate in patients with malignant thyroid cancer, whereas lower doses of the hormone may be used in patients with benign, or nonmalignant, growths of the thyroid gland. The long-term effects of L-thyroxine treatment require further study. (Consumer Summary produced by Reliance Medical Information, Inc.)

Editorial, Measurement, Thyrotropin

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Measurement of bone mineral density: current status

Article Abstract:

Osteoporosis is a condition of reduced bone density that increases the risk of bone fractures. Bone densitometry is used to measure bone mineral density, in order to identify people at risk for osteoporotic fractures and to monitor the progress of therapeutic treatments. Single-photon absorptiometry (SPA) was the first commercially available noninvasive method for measuring bone mineral density. SPA is performed on the forearm submerged in water. A beam of photons containing a radioactive substance (usually iodine-125) is directed at the forearm. As bone density increases, the passage of the photon beam through the bone decreases. SPA is simple to perform and exposure to radiation is low. Dual-photon absorptiometry (DPA) is used to measure bone density in the spine, hip and total body. When both SPA and DPA were performed in women during early menopause, greater decreases in bone density were observed in the spine than in the forearm (radius). Radiation exposure during DPA is low, but it is greater than during SPA. Because X-ray tubes are more advantageous to use than decaying isotopes, all previous manufacturers of DPA systems have switched to dual X-ray absorptiometry (DXA). DXA offers a high level of precision in measuring bone density in the spine, hip and total body, and increases the ability to monitor changes in bone density in patients. Radiation exposure with DXA is similar to SPA. Quantitative computed tomography (QCT) is the only noninvasive technique that gives a true measure of the three-dimensional density of the bone. It can be performed by most CT scanners and is the only method that can measure bone density at any skeletal site in the body. QCT is the most sensitive method for discriminating between patients with spinal osteoporosis and normal patients. However, QCT is expensive and exposes the patient to higher levels of radiation than the other techniques. (Consumer Summary produced by Reliance Medical Information, Inc.)

Methods, Diagnosis, Medical examination, Bone densitometry

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Skeletal integrity in premenopausal and postmenopausal women receiving long-term L-thyroxine therapy

Article Abstract:

Thyroxine is used to restore deficient levels of this thyroid hormone in patients with hypothyroidism (decreased function of the thyroid gland) or to suppress abnormal thyroid growth. Thyroid hormone stimulates bone resorption, a process in which the bone is dissolved and absorbed into tissue, leading to bone loss and increased risk of fracture. Studies have shown that suppressive L-thyroxine treatment causes bone loss, which is most likely due to the use of excessive doses of the hormone. If the levels of L-thyroxine are maintained within normal or physiologic limits by continuous monitoring of L-thyroxine treatment (as indicated by the free thyroxine index, or FT4I), the toxic effects on the bone may be reduced. The notion that maintaining physiologic levels of thyroid hormone would reduce bone loss was assessed in 28 women who were premenopausal, or had not stopped menstruating, and 28 postmenopausal women. The duration of L-thyroxine treatment was 12 years in most premenopausal women and 15 years in most postmenopausal women. The bone density of the hip and spine were measured, and L-thyroxine treatment was monitored to maintain physiologic FT4I levels. These values were normal in 79 percent of premenopausal women and 86 percent of postmenopausal women. Premenopausal women had lower bone densities of the hip and spine, compared with untreated women of similar age. Postmenopausal women also had lower bone densities of the hip, compared with untreated women of similar age. When patients with Graves' disease (goiter; enlargement of the thyroid gland) were excluded, the differences in bone density of the hip between L-thyroxine-treated and untreated women were not apparent. These findings show that bone loss is reduced by carefully monitoring L-thyroxine treatment and maintaining thyroxine levels within physiologic range. (Consumer Summary produced by Reliance Medical Information, Inc.)

Physiological aspects, Bone resorption, Postmenopausal women, Graves' disease

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