Thyroid hormone and bone

The role of FSH and TSH in bone loss and its clinical relevance

Osteoporosis, a global health problem, is now frequently recognized to be secondary to alterations in the pituitary-bone axis. This review examines the current evidence for how dysregulation of the pituitary-bone axis leads to osteoporotic bone loss. Specifically, perimenopausal bone loss in the context of follicle-stimulating hormone action, and hyperthyroid bone loss in the context of thyroid-stimulating hormone action are explored. From the reviewed scientific findings, recommendations for early diagnosis and better clinical management of bone loss are made.

New horizons in skeletal physiology and pathophysiology

OBJECTIVE

To review new discoveries that revisit our current thinking on the genesis of osteoporosis using hypogonadal and thyrotoxic bone loss as examples.

METHODS

We focus on cell biologic, mouse genetic, and human studies that have established a direct action of the interior pituitary hormones follicle-stimulating hormone and thyrotropin on the skeleton and discuss emerging clinical evidence for a novel pituitary-bone axis in humans that bypasses master endocrine organs, namely the ovaries and thyroid gland.

RESULTS

The cataloguing of human mutations, the use of genetically modified mice that recapitulate human disease, and the rapid growth of genomic sciences have together had a profound impact on how basic research is translated into clinical practice. The skeleton has become a paradigm for the application of such advances to an extent that hitherto unrecognized physiologic and pathophysiologic findings have emerged. We propose that hypogonadal and thyrotoxic bone loss are not solely due to changes in the level of master hormones, but instead also arise from the direct action of anterior pituitary hormones on the skeleton.

CONCLUSIONS

We predict a pituitary-bone axis in which pituitary hormones bypass traditional endocrine targets to affect the skeleton directly with remarkable sensitivity. New therapeutic targets thus become a likely possibility.

Role of the pituitary-bone axis in skeletal pathophysiology

PURPOSE OF REVIEW

Embedded within textbooks for decades is the hard fact that releasing hormones from the anterior pituitary, namely, follicle-stimulating hormone, thyroid-stimulating hormone and adrenocorticotropic hormone, stimulate master hormone secretion from target endocrine organs. We propose a paradigm shift in endocrine physiology, which is that these hormones act by design on bone directly, also now considered an endocrine organ.

RECENT FINDINGS

Complementary investigations using mouse genetic and cell biological approaches reveal that follicle-stimulating hormone and thyroid-stimulating hormone act on bone cells directly to regulate bone remodeling and bone mass. Thyroid-stimulating hormone inhibits bone remodeling, whereas follicle-stimulating hormone stimulates it. We also find that the posterior pituitary hormone oxytocin is anabolic to the skeleton.

SUMMARY

An ambitious extrapolation is that a plurality of pituitary hormones acts in concert as part of a 'pituitary-bone' axis to regulate skeletal integrity in health and disease. When dysregulated master hormone levels during hypogonadism and hyperthyroidism cause altered pituitary hormone secretion through hypothalamic feedback, the latter hormones contribute to the skeletal loss.

Thyroid-stimulating hormone, thyroid hormones, and bone loss

It has become accepted by virtue of rich anecdotal experience and clinical research that thyrotoxicosis is associated with high-turnover osteoporosis. The bone loss, primarily due to accelerated resorption that is not compensated by a coupled increase in bone formation, has been attributed solely to elevated thyroid hormone levels. Evidence using mice lacking the thyroid hormone receptors alpha and beta establishes a role for thyroid hormones in regulating bone remodeling but does not exclude an independent action of thyroid-stimulating hormone (TSH), levels of which are low in hyperthyroid states, even when thyroid hormones are normal, as after thyroxine supplementation and in subclinical hyperthyroidism. We show that TSH directly suppresses bone remodeling and that TSH receptor null mice have profound bone loss, suggesting that reduced TSH signaling contributes to hyperthyroid osteoporosis. TSH and its receptor could become valuable drug targets in treating bone loss.

TSH and Bone Loss

We have recently challenged the view that the bone loss associated with hyperthyroidism is solely due to elevated thyroid hormone levels. We find that thyroid-stimulating hormone (TSH), derived from the anterior pituitary gland, inhibits bone resorption by the osteoclast. Mice haploinsufficient in the TSH receptor show reduced bone density and evidence of enhanced bone resorption in the face of normal thyroid function. In humans, TSH inhibits markers of bone resorption with a single administration, and low TSH levels correlate with increased fracture risk. The evidence that low TSH levels predispose to osteoporosis in hyperthyroidism is discussed in view of the emerging role of pituitary hormones in bone biology.