Correlation between thyroid sensitivity indices and bone metabolism in newly diagnosed middle aged and elderly...

To investigate the association between thyroid hormone sensitivity indices and bone metabolism markers in newly diagnosed middle-aged and elderly type 2 diabetes mellitus (T2DM) patients with normal thyroid function.

We retrospectively analyzed 350 newly diagnosed T2DM patients (≥ 45 years), stratified by bone mineral density into Group A (normal bone density group) and Group B (low bone mass and osteoporosis group). General data and clinical biochemical parameters were collected: free triiodothyronine (FT3), free thyroxine (FT4), thyroid-stimulating hormone (TSH), 25-hydroxyvitamin D (25(OH)D), parathyroid hormone (PTH), osteocalcin (OC), bone-specific alkaline phosphatase (BALP), serum calcium (Ca), serum phosphorus (P), fasting plasma glucose (FPG), glycosylated hemoglobinA1c (HbA1c), total cholesterol (TC), triglyceride (TG), low-density lipoprotein cholesterol (LDL-C), high-density lipoprotein cholesterol (HDL-C), serum creatinine (SCr), serum uric acid (SUA), and estimated Glomerular Filtration Rate (eGFR). Thyroid sensitivity indices calculated were: thyrotropin resistance index (TT4RI), TSH index (TSHI), the ratio of FT3/FT4, and the secretory capacity of the thyroid (SPINA-GT). Associations were assessed using multiple linear regression, binary logistic regression, and stratified analysis.

Group B patients had lower BMI, DBP, FT3, FT3/FT4 ratio, FPG, HbA1c, TG, SCr and SUA and higher age, female proportion, OC and HDL-C (P < 0.05). Spearman Correlation analysis showed that 25(OH)D was positively associated with FT3/FT4 ratio; Ca was positively associated with FT3; and P was associated with TSH, TT4RI and TSHI and negatively associated with SPINA-GT (P < 0.05). Multiple linear regression analysis showed that FT3, FT3/FT4 ratios were positive factors for serum OC, BALP and Ca, and FT4 was an inverse correlation with serum BALP (P < 0.05). Binary Logistic regression analysis showed that the elevated FT3/FT4 ratio was a protective factor for the occurrence of low bone mass and osteoporosis (P < 0.05). Stratified by HbA1c, the effect of FT3/FT4 ratio on low bone mass and osteoporosis showed that the FT3/FT4 ratio was inversely associated with the occurrence of low bone mass and osteoporosis in the HbA1c < 7% group (P < 0.05). Simple effect analysis of the interaction between FT3/FT4 ratio, HbA1c and bone density showed that T2DM patients with HbA1c ≥ 7% had a lower FT3/FT4 ratio and a higher risk of developing low bone mass and osteoporosis compared to those with HbA1c < 7%.

In newly diagnosed middle-aged and elderly T2DM patients with normal thyroid function, the peripheral sensitivity to thyroid hormone indix FT3/FT4 ratio was significantly associated with serum OC, BALP, and Ca levels. The lower FT3/FT4 ratio may increase the risk of low bone mass and osteoporosis in newly diagnosed T2DM patients with normal thyroid function, and the reduced index may serve as a tool for early screening and a predictor of low bone mass and osteoporosis.