A patient presents with a tender thyroid and symptoms of hyperthyroidism following a viral infection. What is the likely diagnosis?

The presentation of a tender thyroid gland along with symptoms of hyperthyroidism following a viral infection is highly indicative of subacute thyroiditis, also known as de Quervain's thyroiditis. This condition typically arises after a viral infection, such as mumps or influenza, leading to inflammation of the thyroid gland.

Patients often experience neck pain and tenderness, and the hyperthyroid symptoms can occur due to the release of thyroid hormones from the damaged thyroid tissue into circulation. The tenderness and recent viral illness align well with this diagnosis.

In contrast, Hashimoto's thyroiditis is an autoimmune condition that usually leads to hypothyroidism rather than hyperthyroidism, and it typically does not present with acute tenderness of the thyroid. Graves' disease is an autoimmune hyperthyroid condition characterized by a diffuse goiter and elevated thyroid hormones, but it is not associated with significant tenderness or recent viral infections. Toxic adenoma, while it can cause hyperthyroidism due to overproduction of thyroid hormones, does not commonly present with tenderness or follow viral illness.

Thus, the combination of a tender thyroid, hyperthyroidism symptoms, and a preceding viral infection strongly supports the diagnosis of subacute thyroiditis.