Reference intervals of serum thyroid function tests in a previously iodine-deficient area

Aim: We undertook the present study to establish reference data for serum thyroid function tests in a previously iodine-deficient area. Methods: Data from 4298 individuals, 20-79 years of age were available for the present analysis. Thyroid function (thyrotropin [TSH], free triiodothyronine [FT3] and free thyroxine [FT4]) and serum autoantibodies to thyroperoxidase (anti-TPOAb) were evaluated from blood samples. Thyroid structure and size were measured by ultrasound. Results: A reference population was selected comprising 1488 persons (825 men) by excluding subjects with known thyroid diseases, and with yet unknown thyroid disorders such as goitre, inhomogeneous thyroid pattern, nodules, hypoechogenicity and anti-TPOAb seropositivity. Reference intervals for serum TSH, FT3, and FT4 were 0.25-2.12 mIU/L, 3.8-7.0 pmol/L, and 8.3-18.9 pmol/L, respectively. Reference serum TSH levels were not comparable to the reference values that were recently established for the U.S. population and most reference values slightly differed from the reference values provided by the manufacturers. Conclusions: The reference ranges of thyroid function tests in this formerly iodine-deficient region are distinct from the reference ranges that were established in areas with iodine sufficiency. Creating a reference population in the present setting should include thyroid ultrasound in order to exclude yet undiagnosed thyroid disorders.