Some depressed patients may suffer from restricted central hypothyroidism, which could occur if levels of the thyroid hormone transport protein transthyretin were low. CSF transthyretin was measured in eight inpatients with refractory major depression and nine neurological patients. The depressed patients had significantly lower transthyretin levels than the comparison subjects, suggesting that central hypothyroidism, with normal peripheral thyroid concentrations, could occur in some depressed patients.