Objective: Results of cohort studies on the association between decreased serum TSH levels and mortality are conflicting. Some studies demonstrated an increased mortality risk in subjects with decreased serum TSH levels, others did not. Even meta-analyses revealed contradictory results. We undertook the present study to investigate the association between decreased serum TSH levels and mortality in the large population-based Study of Health in Pomerania (SHIP).
Design and methods: Data from 3651 individuals from SHIP without known thyroid disorders or thyroid treatment were analyzed. Serum TSH, free triiodothyronine, and free thyroxine levels were determined by immunochemiluminescent procedures. Decreased TSH was defined as serum TSH levels below 0.25 mIU/l. Cox regression was used to associate decreased TSH levels with mortality.
Results: The median duration of follow-up was 8.5 years (30 126 person years). During follow-up, 299 individuals (6.9%) died corresponding to a death rate of 9.92 deaths per 1000 person years. Survival time was shorter in subjects with decreased serum TSH levels compared to euthyroid individuals. After adjustment for age and sex, however, there was no association between decreased serum TSH levels and all-cause mortality (hazard ratio: 0.95; 95% confidence interval: 0.67; 1.36). Likewise, decreased serum TSH levels were neither associated with cardiovascular nor with cancer mortality.
Conclusions: There is no independent association of decreased serum TSH levels with all-cause, cardiovascular, and cancer mortality in the adult northeast German population. Although our study has some strengths, we cannot finally conclude on therapeutical implications in individuals with subclinical thyroid diseases.