We studied 23 elderly (other than 65) and 44 younger ambulatory adult patients with primary hypothyroidism to ascertain the dose of levothyroxine needed for complete replacement in relation to age. The elderly patients (average age, 75.7 years) needed an average 118 microgram/d whereas the younger patients (average age, 48.1 years) needed 158 microgram/d. The data indicate that thyroxine requirement may decrease continuously with age. This decrease probably reflects the progressive decrease in thyroxine degradation rate that occurs with age.