The association of thyroid hormone levels and incidence of chronic kidney disease: the Tehran thyroid study (TTS).

BACKGROUND

Evidence regarding the relationship between thyroid hormone levels within the normal range and the incidence of chronic kidney disease (CKD) in adults is scarce. This study aimed to identify the association between thyrotropin (TSH) and free thyroxine (FT4) levels with the incidence of CKD in a large cohort study over long-term follow-up.

METHODS

This prospective cohort study, with an 18-year follow-up, included 4118 adults without CKD from the Tehran thyroid Study (TTS). Participants were categorized by tertiles of normal TSH levels (low-normal, middle-normal, and high-normal) and abnormal TSH. The study outcome was incident CKD, defined as an estimated glomerular filtration rate (eGFR) < 60 mL/min/1.73 m. Multivariable Cox proportional hazard models were used to estimate hazard ratios (HRs) for CKD incidence based on thyroid hormone levels.

RESULTS

The HR for CKD development was 1.08 (95%CI: 1.01-1.15) per 1 SD increase in the TSH levels. Compared with participants with low-normal TSH levels, those with high-normal (HR:1.37; 95%CI: 1.03-1.84) and abnormal TSH (HR:1.24; 95%CI: 1.05-1.46) had a significantly higher risk of developing CKD. In subgroup analyses, the association between TSH level and CKD was significant in participants younger than 60 years, females, non-obese, non-smokers, and those without diabetes and hypertension. No association was observed between FT4 levels and incident CKD (HR: 0.92; 95%CI: 0.79-1.09). However, a significant association was observed between FT4 levels within the normal range and CKD development in those younger than 60 years old (HR: 0.77; 95% CI: 0.61-0.98).

CONCLUSION

Increased TSH levels, even within the normal range, linearly increased the risk of CKD even after adjustment for important risk factors. As a result, TSH may potentially be an independent risk factor for incident CKD.