Differential Diagnosis for High T3 with Normal T4 and TSH

Single Most Likely Diagnosis

• T3 Toxicosis: This condition occurs when there is an excess of T3 in the body, but T4 and TSH levels are within the normal range. It can be caused by the ingestion of excessive amounts of T3, often seen in patients taking thyroid hormone replacement or supplements. The high T3 level can suppress TSH production, but if the T4 level is normal, it suggests that the thyroid gland itself is not overproducing thyroid hormones.

Other Likely Diagnoses

• Thyroiditis: Inflammation of the thyroid gland can cause the release of stored thyroid hormones, including T3, into the bloodstream, leading to elevated T3 levels. However, T4 and TSH might remain within the normal range, especially in the early stages of the condition.
• Heterophile Antibodies: The presence of heterophile antibodies can interfere with the accurate measurement of TSH, potentially leading to falsely normal TSH levels in the context of elevated T3. This is more of a laboratory issue rather than a direct thyroid dysfunction.
• Pituitary TSH Secretion Disorder: Although rare, disorders affecting the pituitary gland's secretion of TSH can lead to abnormal thyroid function tests, including elevated T3 with normal T4 and TSH.

Do Not Miss Diagnoses

• Thyroid Storm: Although TSH is normal, a thyroid storm is a life-threatening condition that requires immediate attention. It can present with elevated T3 levels, among other symptoms like tachycardia, fever, and altered mental status. Missing this diagnosis could be fatal.
• Metastatic Thyroid Cancer: Rarely, metastatic thyroid cancer can produce T3, leading to its elevation while T4 and TSH remain normal. This is a critical diagnosis not to miss due to its implications for treatment and prognosis.

Rare Diagnoses

• Familial Dysalbuminemic Hyperthyroxinemia (FDH): A rare condition characterized by an abnormal albumin that binds T4 with high affinity, potentially leading to elevated free T3 levels due to decreased binding of T3 to this abnormal albumin. T4 and TSH levels can remain normal.
• Resistance to Thyroid Hormone (RTH): A rare genetic disorder where there is reduced sensitivity to thyroid hormones. This can lead to elevated levels of T3 (and sometimes T4) with normal or slightly elevated TSH levels, as the body attempts to compensate for the perceived lack of thyroid hormone effect.
• Struma Ovarii: A rare ovarian tumor that can produce thyroid hormones, potentially leading to hyperthyroidism with elevated T3 levels. However, T4 and TSH might remain within the normal range, depending on the tumor's secretion patterns.