Granulomatous Thyroiditis Diagnosis

The patient's presentation suggests Granulomatous thyroiditis (also known as de Quervain's thyroiditis).

Clinical Presentation Analysis

• Viral Prodrome: Symptoms like fever, malaise, and neck pain occurring a few days after a viral infection are characteristic of Granulomatous thyroiditis.
• Thyroid Findings: A firm enlargement of the thyroid lobes is typical.
• Lab Results: Normal thyroid antibodies rule out Hashimoto's thyroiditis. A high-normal serum T4 level can indicate the transient hyperthyroid phase sometimes seen in Granulomatous thyroiditis.

Differential Diagnosis Reasoning

• Autoimmune thyroiditis (Hashimoto's): Usually associated with positive thyroid antibodies and often leads to hypothyroidism, not typically acute pain/fever post-viral.
• Lymphoma of thyroid: A rare condition, often presenting as a rapidly growing mass, typically in older individuals or those with pre-existing thyroid issues. Less likely given the viral prodrome.
• Granulomatous thyroiditis: Fits the clinical picture well, including the viral onset, pain, fever, firm enlargement, normal antibodies, and potential transient hyperthyroidism.
• Riedel's thyroiditis: Characterized by a dense fibrous replacement of the thyroid, leading to a very hard, fixed ("woody") gland, often causing compressive symptoms. Less commonly associated with acute fever and viral prodrome.

Therefore, based on the combination of post-viral symptoms, neck pain, fever, firm thyroid enlargement, normal antibodies, and high-normal T4, Granulomatous thyroiditis is the most probable diagnosis.