The prevalence and causes of low serum thyrotropin concentration were studied in 886 individuals at age 85 years (601 women and 285 men). All participants were subject to detailed clinical and biochemical evaluation, including determination of serum thyrotropin and free thyroxine concentrations by chemiluminometric assays. Samples with thyrotropin concentrations below 0.20 mU/L or above 6.0 mU/L and/or free thryoxine concentrations above 22.0 pmol/L were selected for further assays. These selected individuals were followed-up during 3 years. Of 18 individuals without thyroid hormone treatment who had thyrotropin concentrations less than 0.20 pmol/L (13 below 0.10 pmol/L), only two were proved to be hyperthyroid; in another three, hyperthyroidism could not be excluded. The results indicate that most elderly individuals with low serum thyrotropin concentrations are not hyperthyroid and that abnormal thyroxine-binding globulin (in conjunction with drug treatment or nonthyroidal illness) is not a common cause of low thyrotropin concentration.
(Arch Intern Med. 1991;151:549-556)