Juvenile-onset Panhypopituitarism in Dogs
Pituitary dwarfism occurs most frequently in German Shepherds but has been reported in other breeds such as the Spitz, Miniature Pinscher, and Karelian Bear Dog. It is inherited as a simple autosomal recessive trait.
Pituitary dwarfism is usually associated with a failure of the oropharyngeal ectoderm of the cranial pharyngeal duct (Rathke’s pouch) to differentiate into trophic-hormone-secreting cells of the pars distalis. Consequently, the adenohypophysis is not completely developed. The second most common cause is craniopharyngioma, a benign tumor derived from the oropharyngeal ectoderm of Rathke’s pouch. Compared with other types of pituitary neoplasms, these tumors tend to develop in younger dogs. Craniopharyngiomas cause subnormal secretion of growth hormone (GH), which results in dwarfism.
Dwarf pups are indistinguishable from normal littermates up to 2 months old. Subsequently, the slower growth rate compared with littermates, retention of puppy coat, and lack of primary guard hairs gradually become evident. German Shepherds with pituitary dwarfism appear coyote- or fox-like, owing to their small size and soft, woolly coat. Bilaterally symmetric alopecia develops gradually and often becomes complete except for the head and tufts of hair on the legs. Permanent dentition is delayed or completely absent. Closure of the epiphyses is delayed as long as 4 years depending on the severity of the hormonal insufficiency and is caused by deficiencies of both thyroid-stimulating hormone and GH. The testes and penis are small, calcification of the os penis is delayed or incomplete, and the penile sheath is flaccid. The ovarian cortex is hypoplastic, and estrus is irregular or absent. Lifespan is shortened because of the resulting secondary endocrine dysfunction, such as hypothyroidism and hypoadrenocorticism. Puppies with panhypopituitarism often have a shrill bark.
Pituitary cysts fill with mucus and eventually occupy the entire pituitary area, resulting in severe compression of the pars nervosa and infundibular stalk. Craniopharyngiomas are large, solid, cystic areas that extend into the overlying hypothalamus. They may also grow along the ventral aspect of the brain, where incorporation of several cranial nerves results in specific nerve function deficits.
Levels of thyroxine, triiodothyronine, and cortisol are reduced or in the low-normal range. In those animals with an equivocal change in basal hormone level, the responses to challenge by exogenous thyrotropin or adrenocorticotropin are subnormal, owing to the hypoplasia or atrophy of the thyroid gland and adrenal cortex. Other useful diagnostic aids include comparison of height with that of littermates, evidence of delayed epiphyseal closure or dysgenesis on skeletal radiographs, and skin biopsy. Cutaneous lesions include hyperkeratosis, follicular keratosis, hyperpigmentation, adnexal atrophy, loss of elastin fibers, and a loose network of collagen fibers in the dermis. Hair shafts are absent, and hair follicles are primarily in the telogen stage of the growth cycle.
The activity of somatomedin C (insulin-like growth factor 1) is low in dwarf dogs. Intermediate somatomedin C activity is present in phenotypically normal ancestors suspected to be heterozygous carriers. Assays for somatomedin C provide an indirect measurement of circulating GH activity in dogs with suspected pituitary dwarfism. Basal levels of circulating canine GH are reported to be detectable but low (normal range: 1.75 ± 0.17 mg/mL) in pituitary dwarfs and do not increase after a provocative test for secretion via clonidine injection (30 mcg/kg, IV) as they do in healthy dogs. Insulin hypersensitivity has been demonstrated in pituitary dwarf dogs, probably due to a change in insulin receptor numbers or affinity of binding in response to the low level of growth hormone.
Treatment of pituitary dwarfism is limited to hormone supplementation for secondary hypothyroidism (22-44 mcg/kg PO once or twice daily) and supplementation with GH or megestrol acetate to promote GH secretion from mammary tissue in the dog. A dosage of 2.5-5 mg/kg of medroxyprogesterone acetate every 3 weeks until clinical response (usually 2-4 months), then every 6 weeks thereafter, has been studied for the treatment of GH deficiency in dogs. Undesirable side effects such as acromegalic features, pyometra, insulin resistance, and mammary hyperplasia have been observed.
Pituitary dwarfism is a rare, inherited disorder of growth hormone deficiency in the dog.
Clinical signs of proportionate dwarfism and secondary hypothyroidism (alopecia) are the most common seen.
Treatment with progestagens may reverse some of the clinical signs of dwarfism but are associated with some bothersome side effects.
Also see Pet Health content regarding juvenile-onset panhypopituitarism in dogs.