The thyroid gland is the body's major regulator of metabolism. By releasing T4 (thyroxine) into the blood stream (which is then converted to T3) the thyroid gland activates a multitude of chemical reactions throughout the body. From protein synthesis, to cell growth and tissue formation, to thermal control, the thyroid gland plays an integral role in all aspects of body metabolism including the regulation of bone remodeling activity. Thyroxine release into the blood is governed by thyroid-stimulating hormone (TSH) from the anterior pituitary gland in the brain. When thyroid hormone levels are insufficient, the anterior pituitary senses this and releases TSH which in turn stimulates the thyroid to release more T4. When T4 and T3 blood levels rise, the pituitary lowers it's production of TSH. This is called a negative feedback loop. The end result, when the body is running smoothly, is normal metabolism.
There are many hormones that affect bone health: estrogen, parathyroid hormone, cortisol, etc. Thyroid hormones are also involved through their influence on bone cells. In hyperthyroidism, the over-production of thyroid hormones causes bone loss. High levels of T4 and T3 stimulate the bone-destroying osteoclasts and, over time, can lead to osteoporosis. Investigators have known for over 10 years that the level of TSH in the blood also has a negative correlation to bone mineral density (BMD). When TSH levels go below 0.5 mIU/L (as seen in individuals with hyperthyroidsim) bone mineral density is often low. What researchers didn't know was if this connection was simply a reflection of the T4 and T3 stimulation upon the osteoclasts, or if the excessively low TSH was actually having a direct negative effect on bone density.
A recent report by Baliram et al. (2012) in The Journal of Clinical Investigation indicates that TSH does indeed have a direct effect on bone. In this study using mice, the researchers demonstrated that TSH has a direct stimulatory action on the bone building osteoblasts. This finding is extremely important especially in light of the fact that reference ranges for TSH are currently under review. Most doctors consider the normal reference range of TSH to be 0.5 to 5.0 mIU/L. This range may soon be narrowed to .03 to 3.0 mIU/L. By reducing the lower range of normal from 0.5 to 0.3, overly aggressive prescription of thyroid medications will have an adverse affect on patient's bone health.
Least you see high levels of TSH as your ticket to improved bone health, remember that all things need to be in moderation. Elevated TSH above 5.0 mIU/L in patients with hypothyroid also appears to be detrimental to bone health. Too much TSH may over-suppress bone resorption and lead to brittleness of bones and heightened fracture risk. My recommendation, if you are on prescription medication for thyroid control, is to keep TSH levels between 1.0 and 4.5 mIU/L. Use this reference range, along with symptomotology, to help you and your doctor fine-tune your prescription.
What I found most fascinating about this study was that it also yielded evidence that there may be local production of a form of TSH within the bone marrow itself. This finding highlights the incredible dynamic and complex nature of bone. Our skeleton, we are learning, is not only important for structural support and as a reservoir for minerals, but it is a vital active endocrine organ involved in helping to sustain our whole-body health. We need to take special care of our bone health!
[Note: In a condition called subclinical hyperthyroidism, thyroid hormone levels can be within the normal range but TSH abnormally low. This "hidden" condition can severely exacerbate bone loss in postmenopausal women. It's always wise to have your doctor check your TSH.]
Baliram et al. 2012. Hyperthyroid-associated osteoporosis is exacerbated by the loss of TSH signaling. The Journal of Clinical Investigation. doi:10.1172/JCI63948.
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