Thyroid hormone

Thyroid hormone (thyroxine/T4, converted in the body to active T3) controls your metabolic rate, and it also controls how fast your bones remodel. T3 binds to receptors inside both osteoblasts (bone-building cells) and osteoclasts (bone-removing cells), setting the pace of the whole remodeling cycle.

When thyroid hormone levels are too high — whether from an overactive thyroid gland or from a levothyroxine dose that's higher than your body needs — the remodeling cycle speeds up dramatically. The problem is that the bone-removal phase speeds up more than the bone-building phase, so each cycle leaves a little less bone behind than before. Over time this adds up to real bone loss and higher fracture risk.

This is why doctors who prescribe levothyroxine for an underactive thyroid aim to keep TSH (thyroid-stimulating hormone) in a normal range rather than overly suppressed, unless there's a specific medical reason — like certain thyroid cancers — to run it lower. If you take thyroid medication, periodic TSH checks and bone density monitoring are reasonable conversations to have with your prescriber; never adjust your dose on your own.