However, it's not known whether the hyperthyroidism that occurs early in the disease before the thyroid burns out was actually responsible for lowering bone density (JAMA, 1991; 265: 2688-91). One definite area of concern is hypothyroid women who are being given too high doses of thyroxine replacement who become hyperthyroid without obvious symptoms. In those cases, studies have demonstrated that this group do have lowered bone density (Thyroid, 1994; 4 (3): 319-26), as do women on thyroxine replacement doses that send levels of the hormone TSH plunging (Semin Nucl Med, 1995; 25: 205-20).