Differential Diagnosis for Elevated TSH, Free T4, and Free T3

Given the laboratory results of TSH 84, free T4 1.03, and elevated free T3 at 4.25, along with a normal pituitary MRI and normal prolactin levels, the following differential diagnoses are considered:

• Single Most Likely Diagnosis
  • Thyroid Storm or Thyrotoxic Crisis: Although TSH is elevated, which typically suggests hypothyroidism, the elevated free T3 and the clinical context could indicate a thyroid storm, especially if the patient is presenting with symptoms such as tachycardia, fever, and altered mental status. The discrepancy between TSH and free T3 levels might be due to the assay interference or the early phase of thyroid storm where TSH could be transiently elevated before becoming suppressed.
• Other Likely Diagnoses
  • TSH-Producing Pituitary Adenoma (TSHoma): Despite the normal pituitary MRI, a TSHoma could still be considered, especially if the MRI was not of high quality or if the adenoma is very small. TSHomas lead to elevated TSH and free thyroid hormones due to the autonomous secretion of TSH.
  • Resistance to Thyroid Hormone (RTH): This condition is characterized by reduced sensitivity to thyroid hormones, leading to elevated levels of TSH and free thyroid hormones. It's a rare condition but should be considered in the differential diagnosis.
  • Heterophile Antibody Interference: This is a laboratory artifact that can cause falsely elevated TSH levels. It's essential to consider this possibility, especially if the clinical presentation does not match the laboratory findings.
• Do Not Miss Diagnoses
  • Pituitary Apoplexy: Although the pituitary MRI is normal, pituitary apoplexy (a condition characterized by sudden hemorrhage or infarction of the pituitary gland) can sometimes present with normal initial imaging. It's crucial to consider this diagnosis due to its potential for severe consequences, including acute hypopituitarism and even death.
  • Metastatic Disease to the Pituitary: Metastases to the pituitary gland can disrupt normal pituitary function, potentially leading to abnormal thyroid function tests. This diagnosis is critical to identify due to its implications for cancer staging and treatment.
• Rare Diagnoses
  • Familial Dysalbuminemic Hyperthyroxinemia (FDH): A rare condition characterized by abnormal binding of T4 to albumin, leading to elevated free T4 levels but normal TSH. However, the elevated free T3 in this case makes this diagnosis less likely.
  • Thyroid Hormone Resistance due to Thyroid Hormone Receptor Mutations: Similar to RTH, but this would typically present with goiter and other specific clinical features.

Each of these diagnoses requires careful consideration of the clinical presentation, laboratory findings, and potentially additional diagnostic testing to determine the underlying cause of the abnormal thyroid function tests.