Reference Intervals for Serum Thyrotropin: Dependence on the Population Investigated

Summary

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 (ΔTSH) 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 δTSH in all groups but the regression equations expressing the qualitative connection of basal and ΔTSH 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 (> 0.7 mU/1 versus > 0.4 mU/1 in the other groups). In thyroid out-patients and geriatric patients a measurable (> 0.1 mU/1) basal TSH indicated measurable ΔTSH (thus excluding clinical hyperthyroidism), while in seriously ill patients only a basal TSH > 0.2 mU/1 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.

Key words

TSH - Normal range - Different populations