Magnitude and Predictors of Subclinical Hypothyroidism in Individuals With Impaired Glucose Tolerance

The screening for thyroid function abnormalities in asymptomatic individuals remains a topic of debate. The impact of subclinical hypothyroidism (SCH) on glucose metabolism has been less extensively researched. Our study aimed to assess the feasibility of thyroid-stimulating hormone (TSH)-based screening and evaluate the prevalence of SCH among individuals with impaired glucose tolerance (IGT) in a community-based setting.  Methodology: We performed a longitudinal study in individuals with IGT by measuring clinical parameters and TSH at baseline and follow-up measurements at 6 and 12 months.

We included 148 participants with IGT. The baseline mean HbA1c of the participants was 6.0% ± 0.2%, and the TSH level was 3.2 ± 2.1 µIU/mL. In the study, we found that 18 (12.2%) individuals with IGT had baseline TSH values in the SCH range (TSH 5-10 µIU/mL). Individuals with IGT who also had a TSH abnormality had significantly lower HbA1c levels as compared to those who were euthyroid (5.9% ± 0.2% vs. 6.1% ± 0.2%, P < 0.008). Other measurements were similar between the two groups. There was a significant decline in mean HbA1c levels on follow-up (baseline HbA1c 6.1% ± 0.2% vs. follow-up HbA1c 5.4% ± 0.7%; P < 0.001). One year later, 14 individuals (87.5%) with SCH reverted to normal TSH levels without specific thyroxine therapy, while 17 individuals (15.8%) who were initially euthyroid developed TSH elevations into the SCH range, suggesting dynamic fluctuations in TSH levels among individuals. There was, however, no change in mean TSH levels on follow-up (baseline 3.2 ± 1.8 µIU/mL vs. follow-up 2.9 ± 2.3 µIU/mL; P = 0.22). The overall prevalence of SCH at baseline and follow-up was similar (16, 13.2%, vs. 18, 14.9%; P = 0.855).

The study highlights that SCH is a transient condition among individuals with IGT. The majority of these individuals reverted to normal thyroid function after one year without requiring thyroid hormone therapy, underscoring the unstable nature of SCH.