Growth hormone (GH) is produced in the anterior pituitary gland and is normally under the control of the hypothalamus via gonadotropin-releasing hormone (GnRH). Growth hormone is challenging to measure directly for several reasons, both physiologically and from a laboratory perspective. Growth hormone is produced in a pulsatile fashion, is highly influenced by various physiological states, and has a short half-life in the bloodstream. In addition, growth hormone is not well conserved among species making assay availability problematic.

Insulin-like growth factor 1 (IGF-1) is produced primarily in the liver in response to growth hormone stimulation. IGF-1 has a much longer half-life compared to growth hormone, is produced in a non-pulsatile manner, is stable in serum, and is well-conserved between species. These features make IGF-1 a suitable substitute for direct measurement of growth hormone, and IGF-1 results can be expected to reflect growth hormone production in dogs and cats. Serum or plasma may be submitted for IGF-1 testing (20005).

Canine pituitary dwarfism is an autosomal recessive inherited disorder of the pituitary gland that results in decreased or absent growth hormone secretion. Deficiency in other pituitary hormones may occur (e.g., TSH, FSH, LH) with hypothyroidism being the most frequent comorbidity identified. Growth hormone insufficiency is also rarely associated with inflammatory or traumatic conditions with concurrent hypoadrenocorticism possible when secretion of adrenocorticotropic hormone (ACTH) is also affected.

Dogs with pituitary dwarfism present with a small stature (stunted growth with retention of normal proportions of head, trunk, and limb size); thin skeleton with delayed closure of growth plates; delayed dental eruption and retention of deciduous teeth; retention of secondary hairs and lack of primary (guard) hairs; and retained testicles or failure to have estrus cycles.

In general, there is a positive correlation between IGF-1 concentration and the size of the dog with young growing puppies anticipated to have higher values than adult dogs. Healthy large breed puppies may have IGF-1 results above the reference interval, whereas healthy small breed adults may have IGF-1 results below the reference interval established with a general population of adult canines.

Growth hormone insufficiency is supported by finding a low IGF-1 concentration in conjunction with supportive clinical findings and the confidence that no other illness is present. Serum concentrations of IGF-1 may also decrease as a metabolic consequence of negative energy balance and alterations in hepatic function. Low concentrations of IGF-1 have also been observed in puppies (and kittens) with congenital primary hypothyroidism, where serum IGF-1 increased in response to thyroid supplementation. Comparing the IGF-1 concentration in a healthy appearing littermate can be helpful.

In dogs, hypersomatotropism, or excessive secretion of growth hormone, is usually associated with excessive endogenous or exogenous progestogen influence on mammary tissue in dogs. Endogenous progestogens are associated with diestrus or pregnancy in intact female dogs, whereas exogenous progestogen exposure is usually related to administration of long-acting progestogen drugs used to suppress estrus. Hypersomatotropism due to a pituitary neoplasm does occur but is very rarely described in dogs. Diabetes mellitus is a recognized metabolic consequence of hypersomatotropism due to the anti-insulin effects of growth hormone.

Acromegaly refers to physical changes that result from the anabolic or growth-promoting effects of excessive IGF-1. Animals with acromegaly present with broadened head, prognathia inferior with increased interdental space, coarse facial features, enlarged paws, respiratory stridor, and abdominal organomegaly.