Associations between thyroid function and mortality: the influence of age.

OBJECTIVE

The aim of this study was to investigate the influence of age on the association between thyroid function and mortality.

DESIGN

The Nijmegen Biomedical Study is a population-based study, comprising 5816 randomly selected adults of all age groups without previously known thyroid disease.

METHODS

TSH, free thyroxine (FT4) and peroxidase antibodies were measured in 2002-2003. The number of deaths were established in 2012 (median follow-up time 9.4 years).

RESULTS

Subclinical thyrotoxicosis was associated with mortality in subjects aged <65 years (hazard ratio (HR) 2.5, 95% CI 1.1-5.7), but not in subjects aged >65 years. As for thyroid function within the normal range: in the 493 participants aged 80 years or older, an FT4 level in the high-normal range (18.5-22 pmol/l) was associated with a higher mortality in comparison with FT4 levels in the middle range (11.5-15.0 pmol/l): HR 1.7 (95% CI 1.0-2.9). In these elderly, TSH levels within the high-normal range (3.0-4.0 mIU/l) were also associated with a higher mortality in comparison with TSH levels within the middle range (1.0-2.0 mIU/l): HR 1.8 (95% CI 1.0-3.1).

CONCLUSIONS

The relationship between thyroid function and mortality differs according to age. This finding might (partially) explain the discrepant results of previous studies examining the relationship between thyroid function and mortality in different age groups.