Correlation between thyroid sensitivity indices and bone metabolism in newly diagnosed middle aged and elderly patients with type 2 diabetes mellitus with normal thyroid function.

Objective: 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.

Method: 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.

Results: 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%.

Conclusion: 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.