Multinodular Goiter: This condition is characterized by a diffusely enlarged thyroid gland that is firm, nodular, and nontender to palpation. It often results from long-standing iodine deficiency or other factors leading to chronic stimulation of the thyroid gland, causing it to become enlarged and develop multiple nodules over time.

Hashimoto Thyroiditis: Although typically associated with hypothyroidism and sometimes a tender gland, the chronic form of Hashimoto thyroiditis can present with a firm, diffusely enlarged, and nodular thyroid gland that is nontender. The presence of antibodies against thyroid peroxidase (TPO) or thyroglobulin can support this diagnosis.
Graves' Disease: In some cases, especially in the early stages or in patients with a long-standing disease, Graves' disease can present with a diffusely enlarged thyroid gland that is firm and nodular. However, it is usually associated with hyperthyroidism symptoms and exophthalmos.

Thyroid Lymphoma: Although rare, thyroid lymphoma can present with a rapidly enlarging, firm, and nodular thyroid gland. It is crucial to consider this diagnosis, especially in older adults or those with a history of Hashimoto thyroiditis, as it requires prompt and aggressive treatment.
Anaplastic Thyroid Cancer: This is a highly aggressive and rare form of thyroid cancer that can cause rapid enlargement of the thyroid gland, which may be firm, nodular, and fixed. Given its poor prognosis, early recognition is critical.

De Quervain Thyroiditis: This is a rare form of subacute thyroiditis that can cause thyroid gland enlargement, but it is typically tender and associated with pain. However, in some cases, it might present without significant tenderness.
Riedel Thyroiditis: A rare form of chronic thyroiditis characterized by a hard, fibrotic, and sometimes nodular thyroid gland that can be fixed to surrounding tissues. It is a rare condition that can mimic thyroid cancer or other serious diseases.