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

Objective: 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.

Methods: 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.

Results: 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.

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