Differential Diagnosis for Normal TSH but Low FT4

Single Most Likely Diagnosis

• Euthyroid Sick Syndrome: This condition, also known as non-thyroidal illness syndrome (NTIS), is the most common cause of low FT4 with normal TSH in hospitalized patients. It occurs in the context of systemic illness, where the body's stress response alters thyroid hormone metabolism without an actual thyroid disorder.

Other Likely Diagnoses

• Thyroid Hormone Resistance: A rare genetic disorder where the body's tissues are resistant to thyroid hormone, leading to low FT4 levels despite normal TSH.
• Pituitary or Hypothalamic Dysfunction: Certain conditions affecting the pituitary gland or hypothalamus can disrupt the normal feedback loop of thyroid hormone regulation, potentially resulting in low FT4 with normal TSH.
• Medication-Induced Thyroid Dysfunction: Certain medications, such as amiodarone, can affect thyroid function tests, leading to low FT4 levels with normal TSH.

Do Not Miss Diagnoses

• Pituitary Apoplexy: A medical emergency where a pituitary tumor suddenly hemorrhages or infarcts, potentially leading to acute hypopituitarism, which can present with low FT4 and normal or low TSH.
• Traumatic Brain Injury: Severe head trauma can disrupt the hypothalamic-pituitary axis, leading to abnormalities in thyroid function tests, including low FT4 with normal TSH.

Rare Diagnoses

• Familial Dysalbuminemic Hyperthyroxinemia: A rare genetic condition characterized by abnormal thyroid hormone binding proteins, leading to low FT4 levels despite normal TSH.
• Selective Pituitary Thyrotroph Resistance: A rare condition where the pituitary gland is resistant to thyroid hormone feedback, potentially resulting in low FT4 with normal TSH.
• Iodine Deficiency or Excess: Both severe iodine deficiency and excess can affect thyroid hormone production, potentially leading to low FT4 levels with normal TSH in some cases.