Low bone mineral density is related to high physiological levels off T4 not to TSH levels below lower reference range.

Objective 
To determine whether thyroid hormone (free thyroxine (fT4)) rather than TSH is directly related to bone mineral density (BMD).Design Cross-sectional population cohort study of peri-menopausal women.
Methods 
Of a sample of 6846 peri-menopausal Dutch women who participated in an osteoporosis-screening programme, a cohort of 2584 was randomly selected for the assessment of thyroid function (TSH, fT4 and thyroid peroxidase antibodies (TPO-Abs)). TPO-Ab-positive women, with a previous history of thyroid dysfunction, overt thyroid disease, subclinical hypothyroidism, osteoporosis or bilateral oophorectomy and those receiving thyroid hormone or hormone replacement therapy were excluded. Of 1477 eligible women, 1426 had TSH and fT4 within the reference range and 51 had low or undetectable serum TSH. BMD was measured at the lumbar spine and low BMD was defined as <0.937 g/cm2.
Results 
The mean BMD in the 51 women with low or undetectable serum TSH was 0.984 g/cm2 compared with 1.001 g/cm2 in the remaining 1426 (t=0.94, P=0.35); 33% of women with low or undetectable serum TSH had low BMD compared with 34% in 1426 euthyroid women. High fT4 but not low TSH in euthyroid women was related to low BMD by multiple logistic regression corrected for age, BMI and smoking (OR, 1.30; 95% CI, 1.02–1.69).
Conclusions 
Higher fT4 levels within the normal reference range but not low or undetectable serum TSH were independently related to decreased BMD at lumbar spine in peri-menopausal women.