Background: Current serum TSH reference intervals have been criticized as they were established from unselected background populations. A special concern is that the upper limit, which defines subclinical hypothyroidism, is too high. The objective was to redefine the TSH reference interval in the adult Finnish population. The current reference interval for the widely used Abbott Architect method in Finland is 0.4-4.0 mU/L.
Methods: Serum TSH and free T4 concentrations were derived from 606 healthy, non-pregnant, 18-91-year-old Finns from the Nordic Reference Interval Project (NORIP) and the possible effects of age, sex and thyroid peroxidase antibody (TPOAb) status were evaluated.
Results: After excluding TPOAb-positive subjects and outliers, a reference population of 511 subjects was obtained. In the reference population, no statistically significant gender- or age-specific differences in mean TSH (1.55 ± 3.30 mU/L) or TSH reference intervals were observed. The new reference interval was 0.5-3.6 mU/L (2.5th-97.5th percentiles).
Conclusions: The current upper TSH reference limit is 10% too high. A TSH > 3.6 mU/L, confirmed with a repeat TSH sampling, may indicate subclinical hypothyroidism. Differences in ethnicity, regional iodine-intake and analytical methods underline the need for redefining the TSH reference interval in central laboratories in different countries.