Neoplastic or reactive proliferation of the C-cells in the thyroid gland. The neoplastic C-cell hyperplasia is associated with familial medullary thyroid gland carcinoma and multiple endocrine neoplasia type II and IIB. Morphologically, it is characterized by the presence of clusters of intrafollicular C-cells with atypical cytologic features. The reactive C-cell hyperplasia is also known as physiological or secondary C-cell hyperplasia and it is associated with conditions that cause hypercalcemia (e.g., hyperparathyroidism).