This chapter covers pathologic changes affecting the pituitary gland, thyroid glands, and adrenal glands. Parathyroids, the endocrine pancreas, and gonads are not included. The latter equine endocrine organs are either very rarely involved in primary pathologic changes (parathyroids, endocrine pancreas) or have been discussed in other chapters (e.g., Chapter 12, “Diseases of the Reproductive Tract”). It should be mentioned that the equine parathyroids are not located anatomically next to the thyroid glands. For their immediate retrieval when performing the necropsy, anatomy textbooks should be consulted before starting the necropsy.
I. Pituitary gland
Inflammatory changes involving the pituitary gland are very rare and represent either abscesses or secondary involvement from basilar empyema.
Hyperplastic and adenomatous changes in the pars intermedia of the pituitary gland in older horses are responsible for pituitary pars intermedia dysfunction (equine Cushing disease) with clinical manifestations of nonseasonal wavy hair coat, muscle weakness, pendulous abdomen, hyperhydrosis, polydipsia/polyuria, or intermittent episodes of laminitis. These proliferative conditions are due to reduction of dopaminergic effects by the hypothalamus and also may be responsible for insulin resistance. Diagnosis is based on clinical signs and endocrine tests.
Figure 11.1. Horse. Pituitary. Pars Intermedia Adenoma. Pituitary enlargement is clearly visible. The condition leads to Cushing disease.
II. Thyroid gland
Thyroid hypertrophy and goiter in foals are described in Chapter 2, “Diseases of Foals and Juveniles.” Hypothyroidism in foals leads to stunting due to delayed ossification of the skeletal system and to a dull, dry, nonshedding hair coat.
Older horses frequently have small discrete nonfunctional adenomas in the thyroid gland as an incidental finding and without clinical signs. These may measure 1–2 cm in diameter and may be expected in cases of slightly asymmetric thyroid glands at necropsy. Originally thought to be of follicular epithelial origin, they are now classified as C-cell adenomas based on ultrastructural and immunohistochemical findings. Rarely are malignant primary thyroid tumors encountered in the horse.
Figure 11.2. Horse. Thyroid. Adenoma. This whitish to tan bulging nodule was an incidental finding at necropsy.
Figure 11.3. Horse. Thyroid. Adenoma and Carcinoma. The adenoma on left is small and well delineated. The carcinoma on right is large replacing the normal brown thyroid parenchyma.
Figure 11.4. Horse. Thyroid. Carcinoma with Mediastinal Metastases. Tumor growths are present in the cranial thorax along the mediastinum (arrow). Ectopic thyroid carcinoma is a differential diagnosis.
Figure 11.5. Horse. Thyroid. Carcinoma. Thyroid C-cells cells have invaded a blood vessel. (H&E)
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