We retrospectively analyzed the cases of 148 febrile patients whose body temperature (BT) was ≥ 37.5°C at our hospital. We categorized them into seven groups; those with bacterial and viral infection, nonspecific inflammation, neoplasm, connective tissue disease (CTD), drug-induced disease, and unidentified causes. Our analysis revealed that the patient's BT at the 1st visit (BT-1st visit) and highest BT during the febrile period (BT-max) differed significantly among all categories except neoplasm. The greatest difference between BT-1st visit and BT-max was highest in the CTD group (1.5°C). Positive correlations of heart rate and C-reactive protein (CRP) level with BT-max and a negative correlation between serum sodium level with BT-max were uncovered. The serum thyroid-stimulating hormone (TSH) level and the ratio of TSH/free thyroxine were negatively correlated with BT-max, especially in the viral infection group, suggesting the existence of occult thyrotoxicosis in accord with a febrile condition, possibly leading to febrile tachycardia. A febrile gap between BT-1st visit and BT-max (except in the neoplasm group) was shown, in which BT-max was correlated with thyroid function. Clinicians should recognize the fluctuation of BT when diagnosing febrile patients, and tachycardia in such febrile patients may be, at least in part, associated with subclinical thyroid dysfunction.
Keywords: C-reactive protein; body temperature; fever of unknown origin; tachycardia; thyroid dysfunction.