Chronic thyrotoxic myopathy development is associated with thyroid hormone sensitivity index, predicted by lower-limb fatigue and the squat-up test.

Thyroid hormones (THs) may affect chronic thyrotoxic myopathy (CTM). The relationship between TH sensitivity and CTM is inconsistent. We aimed to investigate the associations between TH sensitivity and the risk of CTM and to screen potential CTMs with strength and function tests. A total of 162 Chinese patients (36.58% men) with Graves' disease were enrolled and divided into CTM and non-CTM groups. TH and sensitivity indices were measured. Muscle power and function were assessed by grip, upper-limb fatigue (ULFT), lower-limb fatigue (LLFT), and squat-up (SUT) tests, and walking pace. Association between sensitivity to TH indices and the risk of developing CTM was assessed via multivariate logistic regression. The diagnostic effectiveness of muscle power and function for predicting CTM was evaluated via receiver operating characteristic (ROC) curves. Thyroid feedback quantile-based index FT3 (TFQIFT3) and the parametric TFQIFT3 (PTFQIFT3), TFQIFT4, and PTFQIFT4 were positively associated with CTM risk by using inverse probability of treatment weighting multivariate logistic regression. For each 1-SD increase in TFQIFT3 and PTFQIFT3, TFQIFT4 and PTFQIFT4, the odds ratios for CTM were 1.67 (95% CI = 1.17-2.48) ,1.64 (95% CI = 1.51-2.93), 1.60 (95%CI = 1.12-2.32), 1.58 (95%CI = 1.11-2.30), respectively. LLFT and SUT best predicted male/female CTM, respectively (AUC = 0.89/0.85). In Graves' disease patients, TH sensitivity is associated with CTM development, which can be predicted by SUT and LLFT results.