Unexplained Low-Grade Fever: Causes and Next Steps - Internal Medicine

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Understanding unexplained low-grade fever can be quite challenging, especially when the symptoms persist despite normal laboratory results. Low-grade fever, defined as a body temperature ranging from 37.3°C to 37.8°C (99.1°F to 100°F), can be attributed to various factors, including infections, inflammatory conditions, and even psychological stressors.
In your case, the recurrent low-grade fever occurring primarily in the evenings, accompanied by sensations of heat in the head and feelings of heaviness or dizziness, suggests a few potential underlying causes. Given that you have undergone multiple tests with normal results, it is essential to consider both common and less common etiologies.

1. Infectious Causes: While you have ruled out common infections through blood and urine tests, some infections can be elusive. For instance, certain viral infections may not always present with elevated inflammatory markers in blood tests. Conditions like Epstein-Barr virus (EBV) or cytomegalovirus (CMV) can cause prolonged low-grade fevers without significant lab abnormalities.
2. Autoimmune Disorders: Autoimmune diseases, such as lupus or rheumatoid arthritis, can sometimes present with low-grade fevers and other systemic symptoms. The presence of fatigue, headaches, and the sensation of heat could suggest an underlying inflammatory process. Testing for specific autoantibodies may be warranted if not already done.

3. Endocrine Issues: Thyroid disorders, particularly hyperthyroidism, can cause symptoms like heat intolerance, fatigue, and low-grade fever. A comprehensive thyroid panel could help rule this out.

4. Psychological Factors: Chronic stress, anxiety, or depression can manifest physically, leading to symptoms such as fatigue, headaches, and even low-grade fever. If you have been under significant stress, addressing mental health may be beneficial.

5. Chronic Fatigue Syndrome (CFS): This condition is characterized by persistent fatigue that is not improved by rest and may include low-grade fever, headaches, and cognitive difficulties. If your symptoms align with CFS, it may be worth discussing this with your healthcare provider.

6. Other Considerations: Environmental factors, such as exposure to allergens or irritants, can also contribute to your symptoms. Additionally, lifestyle factors, including sleep patterns, diet, and hydration, should be evaluated.

Given your symptoms and the duration of your condition, it would be advisable to consult a specialist. The most appropriate initial step would be to see an infectious disease specialist to further explore any potential hidden infections. If they rule out infectious causes, a referral to a rheumatologist may be necessary to evaluate for autoimmune conditions.
In the meantime, maintaining a symptom diary could be helpful. Documenting the frequency and timing of your symptoms, along with any associated factors (such as stress levels, sleep quality, and diet), can provide valuable insights for your healthcare provider.
In summary, while unexplained low-grade fever can be frustrating, a systematic approach to diagnosis, including consideration of infectious, autoimmune, endocrine, and psychological factors, can help identify the underlying cause. Don't hesitate to advocate for yourself and seek further evaluations if your symptoms persist.