67. Neural response to transcranial magnetic stimulation in adult thyroid hormone resistance

Thyroid hormone resistance describes a rare syndrome in which the thyroid hormone levels are elevated, but the thyroid stimulating hormone level is not suppressed. The importance of thyroid hormones in brain normal function is corroborated by the frequent association of thyroid dysfunctions with neurological and psychiatric symptoms. In this study, we investigated whether adult thyroid hormone resistance affects cortical excitability and modulates inhibitory and excitatory intracortical circuitries. Cortical excitability was probed with transcranial magnetic stimulation in 6 patients with thyroid hormone resistance, 10 patients affected by hypothyroidism, and 10 age-matched healthy controls. We tested motor thresholds, motor evoked potential recruitment curve, cortical silent period, intracortical inhibition, and intracortical facilitation. Patients with thyroid hormone resistance showed decreased steepness of the motor evoked potential recruitment curves. These changes were paralleled by shorter cortical silent period and increased intracortical inhibition. In conclusion, thyroid hormones may influence cortical excitability and cortical inhibitory circuits in adults.