One of the important and frequently encountered situations in daily medical practice that requires consultation across different medical specialties is the interpretation of thyroid function tests, particularly when laboratory results show an elevated free thyroxine (FT4) level while thyroid-stimulating hormone (TSH) remains within the normal range or is not suppressed. This finding requires careful evaluation because it may be associated with genuine endocrine disorders, medication-related effects, or laboratory interferences and errors that can affect the accuracy of test results. It is important to emphasize that laboratory interference can occur even in highly advanced laboratories worldwide. Therefore, clinicians should not immediately consider rare endocrine disorders before reviewing the more common causes. Attention should also be paid to the timing of medication administration and blood sample collection. First: The Importance of Reviewing Medications and Dietary Supplements Before assuming the presence of a genuine thyroid disorder or suspecting laboratory interference, the patient’s medication history should be carefully reviewed, including all medications and dietary supplements being used, as some may affect thyroid hormone levels or interfere with their laboratory measurement. Important examples across different medical specialties include: 1. Cardiac Patients, Cardiac Wards, and Intensive Care Units: Certain medications commonly used in these settings may affect thyroid function test results, including: • High-dose diuretics. • Heparin, which may cause an apparent elevation in FT4 due to its effects on thyroid hormone binding to proteins. • Amiodarone, which affects thyroid hormone metabolism and may cause alterations in thyroid function test results. 2. Dermatology, Nutrition, and Dietary Supplement Patients: Particular attention should be paid to dietary supplement use, especially biotin, which can interfere with certain laboratory immunoassays, potentially leading to inaccurate thyroid function test results. 3. Patients with Joint Disorders: Some non-steroidal anti-inflammatory drugs (NSAIDs) may affect the binding of thyroid hormones to their carrier proteins, potentially influencing free thyroid hormone measurements under certain conditions. Second: The Importance of Paying Attention to Patients in Intensive Care Units Thyroid function test results in patients admitted to cardiac wards and intensive care units require particular attention because these patients often receive multiple medications simultaneously, including diuretics, anticoagulants, and other cardiac medications. Furthermore, critical illness and its associated physiological changes may affect thyroid hormone levels and laboratory test results. Therefore, interpreting these results without considering the patient’s clinical condition and medication history may lead to inaccurate conclusions. Third: Laboratory Interferences and Errors Laboratory interference is an important factor that should be considered when test results are inconsistent with the patient’s clinical presentation. Such interference may result from certain supplements, antibodies, or other factors that affect the immunoassay techniques used in laboratory testing. In these situations, repeating the test using a different laboratory method or reassessing the results in consultation with the laboratory and treating physician may be necessary, depending on the clinical circumstances. Conclusion and Recommendations When laboratory results show Normal TSH + High FT4, hyperthyroidism should not be immediately assumed. Instead, a systematic diagnostic approach should be followed, including: 1. Reviewing the patient’s medications and dietary supplements. 2. Checking the timing of medication administration in relation to blood sample collection. 3. Considering the possibility of laboratory interference or analytical errors.4. Reassessing thyroid function tests when necessary, with the possibility of using an alternative laboratory measurement method. 5. After excluding medication-related and laboratory causes, proceeding with the evaluation of genuine endocrine disorders and less common differential diagnoses. We hope that our colleagues in Cardiology, Intensive Care, Cardiac Care, Nutrition, Dermatology, Endocrinology, Anesthesia, and Rheumatology will take these points into consideration when requesting and interpreting thyroid function tests. This approach is essential for improving diagnostic accuracy, enhancing the quality of medical practice, and reducing unnecessary diagnoses and avoidable additional investigations. Laboratory results should never be interpreted based on numbers alone; their assessment requires correlation with the patient’s clinical condition, medication history, and the circumstances surrounding sample collection. Al Mustaqbal University The First University in Iraq