Background: Carpal tunnel syndrome (CTS) is a common peripheral entrapment neuropathy in patients with hypothyroid. The diagnosis of CTS is usually clinical and confirmed by electrodiagnostic (EDX) procedures. This study aimed to describe the diagnostic accuracy of high-resolution ultrasonography (US) as an alternative method to nerve conduction study (NCS) for the diagnosis of subclinical CTS in patients with hypothyroidism.Methods: Between April 2013 and November 2014, from the patients with the diagnosis of hypothyroidism referring to the institute of endocrinology and metabolism of Firoozgar Hospital, Tehran, Iran, those who met our inclusion criteria entered this cross-sectional study. The patients divided into two groups of subclinical CTS with the age- and gender-matched control group. US measurements of the median nerve cross-sectional area (CSA) in the CT inlet were compared with the NCS results as the gold standard diagnostic test.Results: A total number of 152 wrists of 76 hypothyroid patients were examined in this study. The mean of median nerve CSA at the tunnel inlet was 9.96 ± 2.20 mm2 for the CTS group and 7.08 ± 1.38 mm2 for the control group (P < 0.05). 31 wrists (20.4%) were diagnosed as CTS using NCS while US diagnosed 19 wrists (12.5%) as CTS. Using receiver-operating-characteristics analysis, the sensitivity and specificity of US at the diagnosis of CTS were 45.0 and 95.8%, respectively, with a median nerve CSA cutoff point of 9.8 mm2. Positive and negative predictive values of US were 87.2 and 85.5%, respectively, with a test accuracy of 85.5%.Conclusion: According to our findings, US has an acceptable diagnostic value to confirm CTS in hypothyroid patients. However, it may not replace NCS due to low sensitivity.