We studied the relationship between age, sex, serum thyroxine concentrations, and serum thyrotropin concentrations in 202 patients with primary hypothyroidism whose ages ranged from 10 to 89 years. The results from two groups of patients were analysed, both combined and separately, by multiple linear regression analysis of the factors age, sex, group, and serum free T4 index or serum T4 concentration, to predict serum TSH. The serum free T4 index or T4 values and age were negatively correlated with the serum TSH concentrations (p<0.001) for all comparisons. In contrast, there was no significant relationship between sex and serum TSH concentrations in these hypothyroid patients. The age-related decline in serum TSH concentrations in hypothyroidism was apparent during adolescence and early adult life (ages 10 to 39 years) and in elderly subjects (ages 61 to 89 years). We conclude that age is a determinant of TSH secretion independent of the level of thyroid secretion.