Influence of age and sex on the concentration of free thyroxin in serum and on the free thyroxin: total thyroxin ratio.

The influence of age and sex on the concentration of free thyroxin and on the free thyroxin/total thyroxin ratio was studied in 104 men and 89 women, ages 15 to 83 years. Free thyroxin gradually declined with age in men but not in women. In addition, sex- and age-related variations for free thyroxin were similar to those for total thyroxin and the two indirect assessments of free thyroxin: the free thyroxin index and the total thyroxin/thyroxin binding globulin ratio. Further, free thyroxin and the free thyroxin/total thyroxin ratio were higher in men than in women. However, the free thyroxin/total thyroxin ratio showed relatively little change with age in either sex. This study suggests that the free thyroxin/total thyroxin ratio is helpful in minimizing the age-related variations in both free and total thyroxin.