Purpose:The aim of this study was to determine whether there are electrodiagnostic differences between carpal tunnel syndrome (CTS) patients with diabetes mellitus, CTS + hypothyroidism (HT), CTS + fibromyalgia syndrome, CTS + rheumatoid arthritis (RA), and idiopathic CTS cases, by comparing nerve conduction studies.Methods:This research examined electrophysiologic studies of 47 untreated HT + CTS, 47 diabetes mellitus + CTS, 49 RA + CTS, 52 fibromyalgia syndrome + CTS, 50 idiopathic CTS cases, and a healthy control group of 50 individuals (a total of 293 patients and 433 hands with CTS).Results:There were no significant differences between the groups in terms of sex and age. There was no significant difference between the CTS groupsin terms of numberswith mild, moderate, and severe CTS. When the CTS groups were compared with the control group, in all CTS groups on both left and right hands, there was a significant prolongation in median motor latency and median sensory latency (in the 3rd finger); also a significant decrease in median sensory velocity in the 3rd finger. In diabetes mellitus, HT, and RA groups, the median motor amplitudes in both hands were significantly decreased compared with the idiopathic group. There was a moderate significant negative correlation between disease duration and median motor amplitudes (of both right and left sides) in RA (right; P = 0.028, r = 0.761, left; P = 0.041, r = 0.694) and HT groups (right; P = 0.035, r = 0.637, left; P = 0.049, r = 0.697).Conclusions:Electrodiagnostic results showed both demyelinating injury and axonal damage in diabetes mellitus, HT, and RA patients with CTS, in these patients during treatment for CTS. Early treatment planning should include the risk factor diseases.