Parathyroid gland

Normal parathyroid gland

The parathyroid gland is composed almost exclusively of chief cells (also called principal cells). These cells are noted in sheets with poorly distinct cell borders and a high N:C ratio.

Parathyroid gland inflammation

Parathyroid gland inflammation has been described histologically but not cytologically. Immune-mediated lymphocytic parathyroiditis leads to diffuse destruction of chief cells and glandular fibrosis. The inflammatory infiltrate is composed of lymphocytes and plasma cells and will eventually lead to hypoparathyroidism (Miller, 2017).

Parathyroid gland neoplasia

Chief cells of the parathyroid gland can form cancerous and hyperplastic lesions, which may lead to a palpable mass in the ventral neck (Kallet et al., 1991) and subsequent aspiration; however, the masses that develop are often too small to be palpated (Feldman et al., 2005; Gear et al., 2005) and are sampled for cytology only after surgical removal. If a cystic lesion is noted within the mass, evaluation of fluid parathyroid hormone levels may be helpful to determine parathyroid origin (Lamoureux et al., 2019; Virtanen et al., 2020). These lesions are often functional, leading to the release of increased amounts of PTH. Common presenting complaints include polyuria and polydipsia, listlessness, muscle weakness, and inappetence (Berger Feldman, 1987). Hypercalcemia (total and ionized) is a common finding with hyperplasia, adenomas, and carcinomas and is often used along with serum phosphorus and measurement of serum PTH to aid in diagnosis (Coady et al., 2019). The median age for diagnosis of primary hyperparathyroidism in the dog is 11.2 years and most dogs diagnosed with hyperparathyroidism are over 8 years of age (Feldman et al., 2005).

Adenomas are more common than hyperplasia or carcinomas (Gear et al., 2005; Parker et al., 2015) and often lead to enlargement of a single parathyroid gland; however, bilateral disease has been documented (Kallet et al., 1991). Lesions are often found in the ventral neck within the thyroid gland and are less common in the cranial mediastinum (Feldman et al., 2005). Cytologically, these lesions contain sheets of cells with poorly distinct cell borders and a high N:C ratio. The cells have a small amount of lightly eosinophilic cytoplasm and a round, central nucleus. Small needle-like, eosinophilic, cytoplasmic granules can be seen but with unknown frequency and unknown significance (Soo Choi Arndt, 2016). The nuclei have lacey to ropey chromatin and occasionally up to three small nucleoli. Only mild anisocytosis and anisokaryosis are observed. Increased nuclear pleomorphism is expected in a parathyroid carcinoma, but overall atypia is still mild.

Parathyroid carcinomas are uncommon. In one study done in collaboration between six veterinary universities, only 25 canine cases were identified over a 20-year period for which complete surgical excision was the treatment of choice (Sawyer et al., 2011). In another collaborative, multi-institutional study, 100 cases were identified over a 10-year period (Erickson et al., 2021). Feline cases are documented only as single case reports or in a small case series (Kallet et al., 1991; Phillips et al., 2003; Cavana et al., 2006; Virtanen et al., 2020). Parathyroid carcinomas rarely lead to a palpable lesion and are often diagnosed after investigation of a persistent hypercalcemia or clinical signs associated with hypercalcemia (Sawyer et al., 2011). Lesions are often identified via cervical ultrasound, and a definitive diagnosis is made via histopathology.