Differential Diagnosis for TSH <0.005 and T4 2.2

Single Most Likely Diagnosis

• Hyperthyroidism: This condition is characterized by an overproduction of thyroid hormones, leading to a suppressed TSH level (<0.005) and an elevated T4 level (2.2). The low TSH and high T4 are consistent with primary hyperthyroidism, where the thyroid gland produces excess thyroid hormones, feedback inhibiting TSH production.

Other Likely Diagnoses

• Thyroiditis: This is an inflammatory condition of the thyroid gland that can cause the release of pre-formed thyroid hormones into the bloodstream, leading to transient hyperthyroidism. TSH is suppressed, and T4 is elevated.
• Exogenous Thyroid Hormone Ingestion: Intentional or accidental ingestion of thyroid hormone medication can suppress TSH production and increase T4 levels.
• Pituitary or Hypothalamic Disease: Certain conditions affecting the pituitary or hypothalamus can lead to decreased TSH production, resulting in low TSH and potentially elevated T4 levels if the thyroid gland is stimulated by other means.

Do Not Miss Diagnoses

• Thyroid Storm: A life-threatening complication of untreated or undertreated hyperthyroidism. Although TSH and T4 levels might not be dramatically different from those in uncomplicated hyperthyroidism, the clinical presentation (severe symptoms such as fever, tachycardia, and altered mental status) is critical for diagnosis.
• Pituitary Apoplexy: A condition where a pituitary tumor undergoes sudden hemorrhage or infarction, potentially leading to acute hypopituitarism, including decreased TSH production. The clinical context (sudden severe headache, visual disturbances) is key to suspecting this diagnosis.

Rare Diagnoses

• TSH-Producing Pituitary Adenoma with Peripheral Resistance to Thyroid Hormone: A rare tumor that secretes TSH, which could be suppressed in the face of high thyroid hormone levels due to peripheral resistance to thyroid hormone action.
• Familial Dysalbuminemic Hyperthyroxinemia: A rare genetic condition characterized by abnormal binding of T4 to albumin, leading to elevated T4 levels with normal free T4 and TSH levels. However, TSH can be suppressed if the condition leads to sufficient feedback inhibition.
• Struma Ovarii: A rare ovarian tumor that contains thyroid tissue and can produce thyroid hormones, leading to hyperthyroidism.