What is the likely diagnosis for a patient presenting with a hard, woody thyroid gland?

The presence of a hard, woody thyroid gland is characteristic of Riedel's thyroiditis, which is an uncommon form of thyroiditis. This condition is associated with a dense, fibrous tissue replacement of the thyroid gland, leading to its firm texture. The fibrosis can extend beyond the thyroid into surrounding tissues, sometimes resulting in compressive symptoms.

In contrast, subacute thyroiditis usually presents with a painful, tender gland and is often associated with a viral infection, leading to an acute inflammatory process rather than a firm, woody consistency. Hashimoto's thyroiditis, an autoimmune condition, can also lead to thyroid enlargement but typically presents as a firm but not woody gland and is often associated with thyroid dysfunction and goiter rather than the characteristic hardening seen in Riedel's. A thyroglossal duct cyst, on the other hand, is a developmental anomaly leading to a midline neck mass that is distinct from thyroid disease and does not involve the thyroid tissue itself. Thus, considering the features of the clinical presentation, Riedel's thyroiditis emerges as the appropriate diagnosis in this context.