Development of a predictive score for post-hemithyroidectomy hypothyroidism using skeletal muscle index, remnant thyroid index, and thyroid-stimulating hormone levels: a retrospective cohort study

Background: Hypothyroidism is a major complication of hemithyroidectomy. Low remnant thyroid volume and high serum thyroid-stimulating hormone (TSH) levels are suggested as risk factors for post-hemithyroidectomy hypothyroidism. Reduced skeletal muscle mass is associated with a variety of postoperative complications. However, its impact on post-hemithyroidectomy hypothyroidism has not yet been studied. This study aimed to evaluate the association between skeletal muscle mass and the onset of post-hemithyroidectomy hypothyroidism and develop a predictive score using skeletal muscle mass in combination with previously reported risk factors. Methods: This study retrospectively analyzed 226 consecutive patients who underwent hemithyroidectomy at Shinshu University Hospital between January 2011 and December 2020. The skeletal muscle area at the fourth thoracic vertebral level and maximal remnant thyroid area were quantified using preoperative computed tomography and standardized by dividing them by the square of the patient's height, designated as the skeletal muscle index (SMI) and remnant thyroid volume index (RTI). Subclinical hypothyroidism was defined as a postoperative elevated serum TSH level (>5 mu U/mL) with a normal free thyroxine (FT4) level (=0.9 ng/dL), overt hypothyroidism as a postoperative increase in serum TSH level (>5 mu U/mL) and a decrease in serum FT4 level (<0.9 ng/dL), and symptomatic hypothyroidism as an elevated serum TSH level (>5 mu U/mL) with hypothyroidism-related symptoms. Logistic regression analysis was used to determine the factors associated with the onset of hypothyroidism. Results: Patients with euthyroid status had significantly higher SMI and RTI than those who developed post-hemithyroidectomy hypothyroidism (SMI, euthyroid: 12.0 +/- 2.4 vs. subclinical hypothyroid: 10.2 +/- 1.7, P<0.001, euthyroid vs. overt or symptomatic hypothyroid: 10.1 +/- 1.7, P<0.001, RTI, euthyroid: 1.19 +/- 0.41 vs. subclinical hypothyroid: 0.92 +/- 0.35, P<0.001, euthyroid vs. overt or symptomatic hypothyroid: 0.84 +/- 0.30, P<0.001). Multivariable analysis demonstrated that low SMI, low RTI [hazard ratio (HR): 3.35, P<0.001], and preoperative high serum TSH levels (HR: 2.54, P=0.003) were independent predictive factors for hypothyroidism. Patients who had low SMI, low RTI, and preoperative high serum TSH levels were more likely to develop hypothyroidism (68.8%) than those with either one (25.3%), two (47.8%), or none (15.2%) of these three factors. Conclusions: Preoperative evaluation of the SMI, RTI, and serum TSH levels may be useful in predicting the development of post-hemithyroidectomy hypothyroidism.