Natural history of subclinical hypothyroidism in diabetes mellitus: a prospective observational study

Background: The natural course of subclinical hypothyroidism (SCH) is variable. Several studies have shown that not all SCH progresses to overt hypothyroidism (OH). Little data are available regarding the natural course of SCH in diabetes mellitus (DM). We aimed to investigate the natural course of SCH in DM patients. Methods: A total of 118 patients with SCH (52 with DM and 66 without DM) were enrolled in this prospective observational study. Anthropometric, thyroid hormone and Anti-thyroid peroxidase (TPO) antibody (Ab) level were measured at baseline. Thyroid hormone levels were also estimated at 6 month and 12 months. Result: The majority of the patients, 76(61.86%), remained with SCH, 21(17.8%) progressed to OH and 24 (20.83%) reverted to normal. The rate of progression to OH (DM: 7.69% vs non-DM: 25.76%) was significantly lower in patients with diabetes than in those without. Multivariate logistic regression analysis showed that the risk factors for progression to OH were the glycemic status and thyroid autoantibody. Conclusion: The present study suggests that patient with SCH have variable disease courses. Glycemic status and autoimmunity are the two most powerful predictors of OH. DM protects the progression to OH whereas thyroid autoantibody increases the risk of progression to OH. We recommend a more aggressive follow up for thyroid autoantibody positive and non-diabetic patients.