The principal hormones of the hypothalamic-pituitary-thyroid (HPT) axis are thyroxine (T) and triiodothyronine (T), the latter being the more potent biologically. Thyroid structure and function undergo subtle changes during the human life cycle. Thyroid hormone deficiency during fetal life does not appear to affect growth and maturation, suggesting that brain development in utero is not significantly thyroid dependent. The earliest sign that patients may develop hypothyroidism is a rise in serum thyroid-stimulating hormone (TSH), associated initially with normal thyroid hormone values. Asymptomatic auto-immune thyroiditis, based on the evidence of circulating thyroid antibodies with normal thyroid function, is common, particularly in older women. In adult women, hypothyroidism results in menorrhagia, anovulation, and an increase in fetal wastage. Peripheral thyroid hormone concentrations were assessed in acutely depressed patients and compared to normal controls, euthymic patients, or both; the results are equivocal. Depression is associated with disturbances in circadian behavioral and biologic rhythms.