Morphologic characteristics of the parathyroid and thyroid glands and serum immunoreactive parathyroid hormone in dogs with pseudohyperparathyroidism.

Hypercalcemia, normo- or hypophosphatemia, and increased urinary calcium clearance were observed in 8 dogs with malignant disease consisting of lymphosarcoma (n = 4), lymphogenous leukemia with bone involvement (n = 2), or carcinoma arising in the perianal region (n = 2). Parathyroid glands from these dogs had a normal or atrophic microscopic appearance. Ultrastructurally, the chief cells of the parathyroid glands had small cytoplasmic areas devoid of secretory granules and containing few organelles associated with hormonal synthesis, which is indicative of an inactive phase of the secretory cycle. In relationship to total serum calcium, serum canine immunoreactive parathyroid hormone was moderately increased in 3 dogs whose parathyroid glands had morphologic characteristics of inactivity. As a result of prolonged hypercalcemia, the thyroid glands contained prominent areas of parafollicular cell hyperplasia. The ultrastructure of a typical hyperplastic parafollicular cells was characterized by a large cytoplasmic area filled with numerous secretory granules and moderate numbers of organelles associated with hormonal synthesis. The results of this study indicate that some nonendocrine malignant neoplasms of dogs may be capable of secreting a hypercalcemia substance immunologically indistinguishable from parathyroid hormone. In the absence of neoplastic involvement of bone, the canine disease has many similarities to pseudohyperparathyroidism.