Pain and small-fiber neuropathy in patients with hypothyroidism

Objective: To investigate large- and small-fiber function in patients with hypothyroidism and pain. Methods: The authors studied 38 women treated for hypothyroidism and with painful extremities and 38 healthy controls. All subjects underwent neurologic examination of the extremities, neurophysiologic testing of large myelinated nerves, and thresholds for warmth detection (WDT), cold detection (CDT), heat-pain detection (HPDT), and cold-pain detection (CPDT) in one upper and both lower limbs. Results: Eighteen patients had ongoing or intermittent ongoing distal pain in their limbs. Of these, 8 reported evoked and 10 reported paroxysmal pain. Fifteen patients had only diffuse musculoskeletal pain. A total of 16 patients had "hyperphenomena" (brush-evoked allodynia, punctate hyperalgesia, or cold allodynia or a combination of these, in their feet or hands or both). Eight patients were classified as having large fiber neuropathy, whereas 20 had "hypophenomena" (elevated thermal thresholds in their feet or hands or both). Thermal thresholds at the feet (WDT, CDT, and HPDT) were elevated (p = 0.001, p = 0.007, and p = 0.003, respectively) in the whole group of patients compared with the controls as well as WDT (p = 0.001) and CDT (p = 0.001) being elevated at the thenar eminence. All patients with ongoing, evoked, or paroxysmal pain had either hyperphenomena or hypophenomena or a combination of the two. Conclusions: Some patients treated for hypothyroidism have symptoms and findings compatible with small-fiber neuropathy or "hyperphenomena" indicating central sensitization.