Assessment of Cardiometabolic Risk Factors and Insulin Sensitivity by Hyperinsulinemic-Euglycemic Clamp in Resistance to Thyroid Hormone β Syndrome.

Background: Resistance to thyroid hormone beta (RTHβ) is a rare disease resulting from mutations in the THRB gene, characterized by reduced T3 action in tissues with high thyroid hormone receptor β expression. Thyroid hormones regulate body composition and metabolism in general, and increased or decreased hormone levels are associated with insulin resistance. This study evaluated the presence of cardiometabolic risk factors and insulin sensitivity in patients with RTHβ. Methods: In all, 16 patients, 8 adults (52.3 ± 16.3 years of age) and 8 children (10.9 ± 3.9 years of age), were compared to 28 control individuals matched for age, sex, and body mass index (BMI). Anthropometry evaluation and blood samples were collected for glycemia, lipids, insulin, interleukin-6 (IL-6), tumor necrosis factor-alpha (TNF-α), leptin, adiponectin, ultrasensitive C-reactive protein (CRPus), free thyroxine, total triiodothyronine, thyrotropin, and anti-thyroid peroxidase measurements. Body composition was assessed using dual-emission X-ray absorptiometry and bioimpedance. Insulin sensitivity was evaluated in adult patients and controls using the hyperinsulinemic-euglycemic clamp (HEC), whereas homeostasis model assessment of insulin resistance (HOMA-IR) was calculated in all individuals studied. Results: Patients and controls presented similar weight, BMI, abdominal perimeter, and total fat body mass. Patients with RTHβ demonstrated higher total cholesterol (TC), p = 0.04, and low-density lipoprotein cholesterol (LDL-C), p = 0.03, but no alteration was observed in other parameters associated with metabolic risk, such as leptin, TNF-α, and CRPus. Two adult patients met the criteria for metabolic syndrome. There was no evidence of insulin resistance assessed by HEC or HOMA-IR. Elevated IL-6 levels were observed in patients with RTHβ. Conclusion: Using HEC as the gold standard method, no evidence of reduced insulin sensitivity in skeletal muscle was documented in RTHβ adult patients; however, higher levels of TC and LDL-C were observed in these patients, which suggest the need for active monitoring of this abnormality to minimize cardiometabolic risk. In addition, we demonstrated, for the first time, that the increase in IL-6 levels in patients with RTHβ is probably secondary to metabolic causes as they have normal levels of TNF-α and CRPus, which may contribute to an increase in cardiovascular risk. A larger number of patients must be studied to confirm these results.