Differential Diagnosis for Low FT4 and Low TSH

Single most likely diagnosis

• Subclinical Hyperthyroidism: This condition is characterized by a low TSH level with a normal or slightly low FT4 level. The provided lab values (FT4: 0.65 ng/dL, TSH: 0.265 uU/mL) are consistent with this diagnosis, as the TSH is low and the FT4 is at the lower end of the normal range or slightly below, depending on the reference range used.

Other Likely diagnoses

• Early or Mild Thyrotoxicosis: In some cases, thyrotoxicosis (a condition of excess thyroid hormones) can present with low TSH and relatively low FT4 levels, especially if the condition is mild or in its early stages.
• Pituitary or Hypothalamic Dysfunction: Abnormalities in the pituitary gland or hypothalamus can lead to decreased TSH secretion, resulting in low TSH levels. The FT4 level can be low if the dysfunction affects the production or regulation of thyroid-stimulating hormone (TSH) significantly.
• Non-Thyroidal Illness (NTI) or Sick Euthyroid Syndrome: Certain systemic illnesses can affect thyroid function tests, leading to low TSH and FT4 levels without actual thyroid disease. This condition is often seen in critically ill patients.

Do Not Miss (ddxs that may not be likely, but would be deadly if missed)

• Thyroid Storm: Although typically presenting with high FT4 levels, in some cases, especially if the patient is taking beta-blockers, the FT4 might not be as elevated as expected. A low TSH with any symptoms suggestive of thyrotoxicosis (e.g., tachycardia, fever, agitation) warrants consideration of this life-threatening condition.
• Pituitary Apoplexy: A medical emergency where the pituitary gland suddenly hemorrhages or becomes infarcted, potentially leading to acute hypopituitarism, which could present with low TSH and FT4 levels among other hormonal deficiencies.

Rare diagnoses

• TSH-Producing Pituitary Adenoma with Peripheral Resistance to Thyroid Hormone: A rare tumor that secretes TSH, which could potentially present with low TSH levels if there's co-existing peripheral resistance to thyroid hormone, though this would more commonly present with elevated TSH.
• Familial Dysalbuminemic Hyperthyroxinemia (FDH): A rare condition affecting the binding of thyroid hormones to albumin, potentially leading to abnormal thyroid function tests, though it typically presents with elevated total T4 and T3 levels but normal free hormone levels.
• Resistance to Thyroid Hormone (RTH): A rare genetic disorder where the body's tissues are resistant to thyroid hormone, which can present with a variety of thyroid function test abnormalities, including low TSH and FT4 levels in some cases.