Differential Diagnosis for Low TSH and High Free T4
Single Most Likely Diagnosis
- Hyperthyroidism due to Graves' Disease: This is the most common cause of hyperthyroidism, characterized by autoimmune stimulation of the thyroid gland, leading to increased production of thyroid hormones (T4 and T3) and subsequent suppression of TSH.
Other Likely Diagnoses
- Toxic Multinodular Goiter: A condition where multiple nodules in the thyroid gland become overactive, producing excess thyroid hormones and suppressing TSH.
- Thyroiditis (Subacute or Silent): Inflammation of the thyroid gland can cause the release of stored thyroid hormones, leading to a temporary state of hyperthyroidism.
- Exogenous Thyroid Hormone Ingestion: Accidental or intentional ingestion of thyroid hormone medication can cause elevated free T4 levels and suppressed TSH.
Do Not Miss Diagnoses
- Thyroid Storm: A life-threatening complication of untreated or undertreated hyperthyroidism, characterized by extreme symptoms such as fever, tachycardia, and altered mental status.
- Pituitary or Hypothalamic Disease: Rarely, a tumor or other disease affecting the pituitary or hypothalamus can cause abnormal regulation of TSH, leading to hyperthyroidism.
- Familial Dysalbuminemic Hyperthyroxinemia: A rare genetic disorder affecting the binding of T4 to albumin, leading to elevated free T4 levels despite normal TSH.
Rare Diagnoses
- Struma Ovarii: A rare ovarian tumor that produces thyroid hormones, causing hyperthyroidism.
- Metastatic Thyroid Cancer: Rarely, metastatic thyroid cancer can produce excess thyroid hormones, leading to hyperthyroidism.
- Iodine-Induced Hyperthyroidism: Excessive iodine intake can cause hyperthyroidism, particularly in individuals with underlying thyroid disease.