The significance of clinical euthyroidism on reference range for thyroid hormones

BACKGROUND: The reference range for thyroid hormones and thyroid stimulating hormone (TSH) is usually established either by using the manufacturer's recommendations or by analyzing blood tests from a serum bank or from the local laboratory staff, assuming that they are healthy and euthyroid. The aim of this study was to examine the significance of clinical euthyroidism and a normal basal oxygen consumption on the reference range for thyroid hormones and TSH. METHODS: A clinical examination, including information on medication, was performed on 31 apparently healthy persons. The following determinations were made for all of the subjects: basal oxygen consumption, serum TSH, serum total T(3), and serum total T(4). T(4) uptake for calculation of FT(4)I was also measured. RESULTS: The clinical examination and determination of VO(2) reduced what initially appeared to be a euthyroid and healthy population by 32%. This also resulted in a narrower reference interval than that suggested by the manufacturer (range) (TSH: 0.35-5.50 mU/l, T(4): 58-140 nmol/l, T(3): 0.9-2.8 nmol/l) or than that established in the initially selected population (TSH=0.76-3.90 mU/l, T(4)=54-168 nmol/l, T(3)=1.27-2.74 nmol/l, FT(4)I=64-123 a.u.). In the finally selected population, this was TSH=0.76-3.90 mU/l, T(4)=59-127 nmol/l, T(3)=1.45-2.74 nmol/l, and FT(4)I=64-120 a.u. CONCLUSION: The method of selecting the population for establishing the reference interval for thyroid hormones and TSH influences the reference ranges. Neglecting to deal with this problem may invalidate screening procedures, especially for subclinical diseases.