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).