Differential Diagnosis for High TSH with Low T3 and T4

Single Most Likely Diagnosis

• Subclinical Hypothyroidism: This condition is characterized by elevated TSH levels with low or normal T3 and T4 levels. It often represents an early stage of hypothyroidism where the thyroid gland does not produce enough thyroid hormones to meet the body's needs, but the condition has not yet progressed to overt hypothyroidism.

Other Likely Diagnoses

• Overt Hypothyroidism: Although typically associated with high TSH and low T3 and T4, some cases might present with these lab findings, especially if the hypothyroidism is due to primary thyroid failure.
• Sick Euthyroid Syndrome (SES): Also known as non-thyroidal illness syndrome (NTIS), this condition occurs in patients with systemic illnesses and can present with abnormal thyroid function tests, including low T3 and T4 with elevated TSH, despite the patient being euthyroid.

Do Not Miss Diagnoses

• Pituitary Tumor: A tumor in the pituitary gland can lead to increased TSH secretion, independent of the thyroid's ability to produce hormones. Missing this diagnosis could lead to delayed treatment of a potentially serious condition.
• Thyroid Hormone Resistance: A rare condition where the body's tissues are resistant to thyroid hormones, leading to elevated TSH and variable levels of T3 and T4.

Rare Diagnoses

• TSH-producing Pituitary Adenoma: A rare type of pituitary tumor that secretes TSH, leading to elevated TSH levels with variable thyroid hormone levels.
• Familial Dysalbuminemic Hyperthyroxinemia: A rare genetic condition affecting the binding of thyroid hormones to proteins in the blood, potentially leading to abnormal thyroid function tests.
• Thyroid Hormone Defects: Rare genetic defects affecting the production or transport of thyroid hormones, which could present with abnormal thyroid function tests, including high TSH with low T3 and T4.