Low FT3 is an independent marker of disease severity in patients hospitalized for COVID-19.

Aim: To study the prevalence of thyroid dysfunction and its association with disease severity in hospitalized patients of coronavirus disease-19 (COVID-19).

Methods: In this retrospective cohort study, thyroid function tests (TFT) of 236 hospitalized patients of COVID-19 along with demographic, comorbid, clinical, biochemical and disease severity records were analysed. Patients were divided into previous euthyroid or hypothyroid status to observe the effect of prior hypothyroidism on the severity of COVID-19.

Results: TFT abnormalities were common. Low free T3 (FT3), high thyroid-stimulating hormone (TSH) and low TSH were seen in 56 (23.7%), 15 (6.4%) and 9 (3.8%) patients, respectively. The median levels of TSH (2.06 vs 1.26 mIU/mL, P = 0.001) and FT3 (2.94 vs 2.47 pg/mL, P < 0.001) were significantly lower in severe disease. Previous hypothyroid status (n = 43) was associated with older age, higher frequency of comorbidities, higher FT4 and lower FT3. TFT did not correlate with markers of inflammation (except lactate dehydrogenase); however, FT3 and TSH negatively correlated with outcome severity score and duration of hospital stay. Cox regression analysis showed that low FT3 was associated with severe COVID-19 (P = 0.032, HR 0.302; CI 0.101-0.904), irrespective of prior hypothyroidism.

Conclusions: Functional thyroid abnormalities (low FT3 and low TSH) are frequently seen in hospitalized patients of COVID-19. Although these abnormalities did not correlate with markers of inflammation, this study shows that low FT3 at admission independently predicts the severity of COVID-19.