Progression of Subclinical Hypothyroidism in a Mexican Public Hospital Population: A Retrospective Cohort Study

Treatment of subclinical hypothyroidism (SCH) is controversial. The uncertainty of the levels of thyroid-stimulating hormone (TSH) that warrant treatment and the risk of progression to overt hypothyroidism may lead to overtreatment. This study aimed to assess the persistence of SCH and its short-term progression to overt hypothyroidism in patients referred to an outpatient endocrinology clinic in Southern Tamaulipas, Mexico, and to identify predictive factors for progression to overt hypothyroidism.

This analytic, observational, and retrospective study analyzed records from 1100 patients at a Mexican public hospital between 2018 and 2019. Exclusion criteria included prior hypothyroidism, levothyroxine use, pregnancy, TSH ≥10.0 mIU/L, age <18 years, and non-completion of follow-up. A final sample of 222 patients with SCH (defined as TSH >4.2 and <10.0 mIU/L, with normal T4 levels) was followed for three months, assessing the regression, persistence, and progression of TSH levels. Statistical analyses included a chi-squared test and Student's t-test. Statistical significance was set at alpha=0.05.

The study included 181 (81.5%) females with a mean age of 49.7 years (±13.5). After three months, 158 (71.2%) patients regressed to euthyroidism, 47 (21.2%) remained subclinically hypothyroid, and 17 (7.6%) progressed to overt hypothyroidism. The highest progression rate to overt hypothyroidism (16.0%) was observed in patients with initial TSH levels >6.0 to ≤8.0 mIU/L (relative risk: 5.4; 95% confidence interval {CI}: 1.83-16.0, p<0.001). Mean baseline TSH levels were 6.57±1.11 mIU/L (95% CI: 6.00-7.15, p<0.001) in those who progressed to overt hypothyroidism. No association was observed between symptoms and disease progression.

Most patients with SCH regressed to an euthyroid state without treatment in the short term, supporting a monitoring-first approach.