Objective
Thyroid status has been implicated in macrovascular disease both in patients with and without diabetes, however its effect on microvascular complications has not been explored. We assessed the prevalence and time to development of diabetic retinopathy in patients with type 2 diabetes with and without thyroxine treatment.
Research design and methods
The prevalence of retinopathy was determined, in a retrospective cohort study from a secondary care referral diabetes clinic patients with type 2 diabetes and coexisting treated hypothyroidism (THD; n=147) and duration matched controls without hypothyroidism (DM2; n=383). Using Kaplan-Meier survival analysis and Cox Proportional Hazards regression model we estimated the time to development of retinopathy in the two groups.
Results
Prevalence of retinopathy was 27.9% in THD group, as compared to 55.1% in the DM2 group (p<0.001). THD were less than one- third as likely to have concurrent retinopathy than, DM2 patients (OR=0.32; 95% CI=0.21 to 0.48; p<0.001). There was a significant difference in the median time to retinopathy between THD and DM2 patients (21.0yrs vs. 13.0yrs: Log-Rank p<0.001). Risk of developing retinopathy in THD patients was two-fifths of that of DM2 patients (hazard ratio=0.418; p<0.001) in a time-dependent variable analysis. The risk of developing retinopathy was concomitantly lower for patients with longer duration of hypothyroidism and thyroxine treatment prior to diagnosis of diabetes (Hazard ratio=0.957:p= 0.004).
Conclusion
A significant sparing effect on development of retinopathy was noted in type 2 diabetic patients with concomitant hypothyroidism (on thyroxine). The exact mechanism(s) for these observations remain to be elucidated
