Differential Diagnosis for Low TSH with Normal T4 and Abnormal Weight Loss in a Male

Single Most Likely Diagnosis

• Subclinical Hyperthyroidism: This condition is characterized by a low TSH level with normal thyroid hormone levels (T4 and T3). It can cause weight loss, among other symptoms, and is a common finding in the context described.

Other Likely Diagnoses

• Thyroid Hormone Resistance: A rare condition where the body's tissues are resistant to thyroid hormones, leading to elevated T4 and T3 levels but with a low TSH. However, in some cases, T4 can be normal, especially in the early stages.
• Non-Thyroidal Illness (NTI) or Sick Euthyroid Syndrome: This condition occurs in patients with systemic illnesses and can present with abnormal thyroid function tests, including low TSH with normal T4, due to the body's response to severe illness rather than a primary thyroid disorder.
• Pituitary or Hypothalamic Disease: Lesions or disorders affecting the pituitary gland or hypothalamus can disrupt the normal feedback loop of thyroid hormone regulation, leading to low TSH levels.

Do Not Miss Diagnoses

• Pituitary Adenoma (Especially TSH-Producing Adenoma): Although rare, a TSH-producing pituitary adenoma could lead to hyperthyroidism symptoms, but the presentation with low TSH and normal T4 is less typical. However, missing this diagnosis could have significant consequences.
• Thyroiditis: In the recovery phase of subacute thyroiditis, TSH can be low with normal or even low T4 and T3 levels. Missing this diagnosis could lead to inappropriate treatment.
• Factitious Hyperthyroidism: Intentional or accidental ingestion of exogenous thyroid hormone can suppress TSH production, leading to low TSH levels with variable T4 and T3 levels.

Rare Diagnoses

• Familial Dysalbuminemic Hyperthyroxinemia: A rare condition characterized by abnormal binding of T4 to albumin, leading to elevated total T4 but normal free T4 levels. TSH can be low due to the feedback mechanism.
• Struma Ovarii: A rare ovarian tumor that can produce thyroid hormones, potentially leading to hyperthyroidism with low TSH and variable T4 levels.
• Iatrogenic Hyperthyroidism due to Amiodarone: Amiodarone, an antiarrhythmic medication, can cause thyroid dysfunction, including hyperthyroidism with low TSH and normal or elevated T4 levels.