Factors influencing serum free T4 in 70-year-old men. Implications for reference intervals in the elderly

We tested a procedure to obtain reference values based on careful clinical analysis of a population sample with the background (a) that there are conflicting reports of age effects on thyroid function parameters, possibly reflecting differences in numbers of sick and drug-treated subjects in the studied populations, and (b) that recommended procedures for the selection of reference subjects in elderly populations may give unacceptably high exclusion rates. Serum free T4 was determined by a ligand-analogue technique in a representative population of 460 males at age 70. The determinations were performed 5 years after the clinical study, and the results were analysed in relation to information obtained at the study as well as during a 5-year follow-up. The free T4 concentration was significantly low in subjects with neoplastic disease and subjects with chronic bronchitis and significantly high in subjects taking non-selective beta-adrenergic blockers. Selective beta 1-blockers had no apparent effect. Information on death during the 5 years after the study proved to have no significance. The free T4 concentration was found to be negatively correlated to body mass and body mass index, a correlation which has previously not been shown. Smoking was associated with a significantly low body mass but had no apparent effect on mean free T4 concentration. A procedure with successive exclusions of clinically defined diagnostic groups with significantly low or high analyte concentration and/or body mass may be used to obtain reference values for thyroid function and may reveal previously unknown relationships.