Wide Spectrum of Thyroid Function Tests in COVID-19: From Nonthyroidal Illness to Isolated Hyperthyroxinemia

Background Changes in thyroid function test (TFT) in COVID-19 patients have been reported in several studies. However, some features such as thyrotoxicosis are inconsistent in these studies. In addition, some drugs such as heparin interfere with the free T4 assay. Objectives This study was designed to examine TFT abnormalities in COVID-19, utilizing direct and indirect methods of free T4 assay. Methods This prospective cross-sectional study was conducted on 131 hospitalized COVID-19 patients. Serum levels of total T3, TSH, T3RU, and total T4 were measured. The free T4 assay was performed using direct (free T4) and indirect (free thyroxin index or FT4I) methods. The patients were categorized into different TFT groups. The clinical characteristics, laboratory findings, and outcomes were compared between the groups. Results The frequencies of nonthyroidal illness (NTI), subclinical/overt hypothyroidism and subclinical/overt thyrotoxicosis were 51.7, 6.9, and 6.9%, respectively. Besides, 6 and 8.1% of the patients had isolated high free T4 and isolated high FT4I without any other TFT abnormality, respectively. The lymphocyte percent was lower in the subclinical/overt group than in other TFT groups (P = 0.002). Atrial Fibrillation (AF) was found in 37.5% of subclinical/overt thyrotoxicosis patients versus 1.7% in the NTI and nil in the other three groups (P < 0.001). Conclusions In addition to the reported TFT abnormalities in COVID-19 in previous studies, some new features like isolated hyperthyroxinemia were found in our study. We found a strong association between subclinical/overt thyrotoxicosis and AF. Regarding the high prevalence of AF in hospitalized COVID-19 patients, request for thyroid function test is rational in COVID-19 patients with this arrhythmia.