Immunohistochemical study of substance P-like immunoreactivity in human thyroid and medullary carcinoma of the thyroid.

Substance P-like immunoreactivity (SPL-IR) was identified by the indirect peroxidase anti-peroxidase (PAP) method in normal thyroid glands as well as in medullary thyroid carcinoma. The SPL-IR cells were visualized intrafollicularly in the thyroid, within cells which resembled the parafollicular C cells which contain calcitonin immunoreactivity. The SPL-IR cells distributed differently in the normal thyroid and in thyroid with secondary C cell hyperplasia.