Differential Diagnosis for Elevated T3 with Normal TSH and T4

Single Most Likely Diagnosis

• T3 Toxicosis: This condition occurs when there is an excess of T3 in the body, which can happen due to various reasons such as exogenous thyroid hormone ingestion, thyroiditis, or a T3-secreting thyroid adenoma. The normal TSH and T4 levels with elevated T3 suggest that the body is attempting to compensate for the excess T3, possibly by reducing the conversion of T4 to T3 or through other feedback mechanisms.

Other Likely Diagnoses

• Thyroiditis: Inflammation of the thyroid gland can lead to the release of pre-formed thyroid hormones, including T3, into the bloodstream, causing transient hyperthyroidism. The normal TSH and T4 levels could be seen in the early stages or during the recovery phase of thyroiditis.
• Exogenous Thyroid Hormone Ingestion: Accidental or intentional ingestion of thyroid hormone supplements can lead to elevated T3 levels. The normal TSH and T4 suggest that the body is not producing excessive amounts of these hormones endogenously.
• Resistance to Thyroid Hormone (RTH): A rare genetic disorder where the body's tissues are resistant to thyroid hormone, leading to elevated levels of T3 to compensate for the resistance. However, this condition typically presents with elevated T4 and TSH as well, but there can be variability.

Do Not Miss Diagnoses

• Thyroid Storm: Although rare, thyroid storm is a life-threatening condition that requires immediate attention. It can present with elevated T3 levels, and even though TSH and T4 might be normal, the clinical presentation of severe hyperthyroid symptoms is key to diagnosis.
• Pituitary Tumor: A tumor in the pituitary gland can lead to excessive production of TSH, which in turn can cause hyperthyroidism. However, in some cases, especially with a T3-secreting adenoma, T3 can be elevated while TSH and T4 remain within normal limits due to negative feedback mechanisms.

Rare Diagnoses

• Familial Dysalbuminemic Hyperthyroxinemia (FDH): A rare condition characterized by an abnormal albumin that has a high affinity for T4, leading to elevated levels of T4 and sometimes T3 due to increased conversion. However, TSH usually remains normal due to the body's attempt to compensate.
• Struma Ovarii: A rare ovarian tumor that contains thyroid tissue, which can produce thyroid hormones, including T3, independently of the normal thyroid gland. This condition can lead to hyperthyroidism with variable effects on TSH and T4 levels.