Chapter 43 - Thyroid Hormone and Bone

Abstract

This chapter focuses on the relationship of the in vitro effects of thyroid hormone on bone cells to the observed effects of the hormone on the skeleton in vivo in both experimental animals and humans. Thyroid hormone is essential for skeletal development and also affects mature bone. Depending on the hormone concentration and stage of life, the effects of thyroid hormone can be either beneficial or deleterious to the skeleton. Thyroid hormones increase IGF-I in osteoblasts and experimental animals; elevated circulating thyroid hormones are associated with increases in IGF-I and IGFBPs in patients and other growth factors, including FGF, are also likely to be involved. Other physiological factors modulate the skeletal effects of thyroid hormones, e.g., estrogens can decrease the deleterious effects of excess thyroid hormone. The pharmacological inhibition of bone resorption with bisphosphonates can also diminish thyroid hormone-stimulated bone loss. Thyroid hormones stimulate osteoblast proliferation, promote the differentiation of this cell type, and stimulate differentiated functions, as shown by increases in alkaline phosphatase activity, osteocalcin expression, and stimulation of collagen synthesis. Thyroid stimulating hormone as an effector of responses in bone is still a subject of ongoing researches and clinical investigations.