Reference Intervals for Serum Thyrotropin: Dependence on the Population Investigated

Abstract

In order to elucidate possible differences of reference intervals in various populations, serum basal thyrotropin (TSH) was measured in euthyroid healthy volunteers (N = 170), in thyroid out-patients (N = 215), in geriatric (N = 354) and in seriously ill (N = 32) patients. The results, except in the healthy control subjects, were compared to the TSH responses (delta TSH) in the Thyrotropin Releasing Hormone test. Normal ranges calculated from the basal TSH of the euthyroid groups of different age were similar. There was a significant positive correlation of basal to delta TSH in all groups but the regression equations expressing the qualitative connection of basal and delta TSH were rather different. The basal TSH cut off point predicting a positive TRH-test (i.e. euthyroidism) with more than 95% probability was higher in the geriatric groups (greater than 0.7 mU/l versus greater than 0.4 mU/l in the other groups). In thyroid out-patients and geriatric patients a measurable (greater than 0.1 mU/l) basal TSH indicated measurable delta TSH (thus excluding clinical hyperthyroidism), while in seriously ill patients only a basal TSH greater than 0.2 mU/l was reliably predictive in this respect. Thus, various populations have different reference intervals for TSH. This fact must be considered when the sensitive TSH is used in different populations as the primary screening parameter for thyroid dysfunctions.