Introduction: The present study aimed to analyze the prevalence of hypothyroidism in patients with rheumatoid arthritis (RA). In addition, the study aimed to elucidate the correlation of hypothyroidism with RA activity and to investigate the relationship between RA and thyroid dysfunction. Materials and Methods: A total of 314 patients were categorized into two groups according to thyroid stimulating hormone (TSH) level: RA without hypothyroidism and RA with hypothyroidism. All patients underwent routine laboratory investigation, including thyroid function testing, and complete clinical assessment. These included the determination of the erythrocyte sedimentation rate as well as the level of TSH, free triiodothyronine, free thyroxine, total triiodothyronine level, total thyroxine level, C-reactive protein, rheumatoid factor immunoglobulin (RF-Ig), RF-IgA, RF-IgG, RF-IgM, cyclic citrullinated peptide immunoglobulin G (CCP IgG), complement component 3, and complement component 4. Based on these data, thyroid function, and rheumatoid factor levels were analyzed. Results and Discussion: Curve estimation using linear regression revealed that CCP Ig level was significantly correlated with the TSH level (r = 0.122, P = 0.031). Conclusion: TSH level may be used as an auxiliary test to assess disease severity in patients with RA and to evaluate thyroid function. This evaluation parameter may be considered for determining clinical prognosis in patients with RA.