Patients who have been on systemic glucocorticoids (GC) for longer than 3 weeks during the previous 12 months will be excluded, as GC may cause suppression of hypothalamic-adrenal axis. In subjects who are on phenytoin, unbound T4 will be measured by equilibrium dialysis, as phenytoin may interfere with direct unbound T4 measurement. Subjects already on L-thyroxine replacement will be excluded.