Differential Diagnosis for a Patient with Normal TSH, Normal T3, and Mildly High T4

• Single Most Likely Diagnosis
  • Thyroid hormone resistance: This condition is characterized by a reduced response to thyroid hormones, leading to elevated T4 levels while TSH and T3 remain normal. The mildly high T4 level in this patient could be indicative of this condition.
• Other Likely Diagnoses
  • Subclinical hyperthyroidism with T4-to-T3 conversion issue: Although TSH is normal, some patients may have a problem with the conversion of T4 to T3, leading to elevated T4 levels.
  • Heterophile antibody interference: The presence of heterophile antibodies can interfere with the measurement of T4, resulting in falsely elevated levels.
  • Thyroid hormone binding protein abnormalities: Alterations in thyroid hormone binding proteins, such as thyroxine-binding globulin (TBG), can affect the levels of T4, leading to elevated measurements.
• Do Not Miss Diagnoses
  • Thyrotoxicosis factitia: This condition involves the ingestion of exogenous thyroid hormone, which can lead to elevated T4 levels. Although less likely, it is crucial to consider this diagnosis due to its potential severity.
  • T4-producing thyroid cancer: Although rare, thyroid cancer can produce T4, resulting in elevated levels. Early detection is critical, making this a "do not miss" diagnosis.
• Rare Diagnoses
  • Familial dysalbuminemic hyperthyroxinemia (FDH): This rare genetic disorder is characterized by an abnormal albumin that binds T4 with high affinity, leading to elevated T4 levels.
  • Thyroid hormone resistance due to thyroid receptor beta (TRβ) mutations: This rare condition is caused by mutations in the TRβ gene, resulting in reduced sensitivity to thyroid hormones and elevated T4 levels.