A popular thyroid medication taken by 23 million Americans may be associated with bone loss, a startling new study finds. Levothyroxine — marketed under brand names such as Synthroid — is the second-most commonly prescribed medication among older adults in the US. It’s consumed by about 7% of the US population. The drug addresses hypothyroidism, also known as an underactive thyroid, by replacing or providing more of a thyroid hormone naturally produced by the body. The study authors, from Johns Hopkins Medicine, linked levothyroxine use to a greater loss of bone mass and bone density over six years — even...