IGF-1 (insulin-like growth factor 1) in cats and dogs

What it shows

Growth hormone is released by the pituitary gland in pulses, its level jumps up and down, and assays for cat growth hormone are no longer available. IGF-1 is easier: it stays in the blood longer, does not pulse, is stable in serum and is similar between species. So a high IGF-1 reflects high growth-hormone activity, and a low one — low activity.

IGF-1 depends on more than growth hormone. The liver needs insulin to make it, so in a cat with insulin-deficient diabetes before insulin is started the value can be falsely low. Poor nutrition and liver disease lower it too, and in dogs it depends strongly on body size and age.

Higher than reference — what it may be a sign of

• Acromegaly (hypersomatotropism) in cats — a pituitary tumour that makes too much growth hormone. It causes diabetes that needs unusually large amounts of insulin, weight gain with more lean mass, a broad face, large paws, a protruding jaw and enlarged organs. In acromegalic cats IGF-1 is dramatically increased.
• Growth-hormone excess in dogs — usually linked to progestogens: the female’s own progesterone in the second half of the cycle or in pregnancy, or progestogen medicines. A pituitary tumour is very rare in dogs.
• Normal growth — growing puppies have higher values than adults, and healthy large-breed puppies may be above the adult range.

Lower than reference

• Pituitary dwarfism in dogs — an inherited lack of growth hormone; IGF-1 activity is low in dwarf dogs. A low value supports the diagnosis only together with matching signs and when no other illness explains it.
• Early diabetes in cats, before insulin is started — lack of insulin lowers IGF-1, which can hide acromegaly.
• Other causes — negative energy balance, liver dysfunction, congenital hypothyroidism in puppies and kittens (in some, IGF-1 rose after thyroid supplementation), and small adult dog breeds, which may sit below the general adult range while healthy.

Look at together with

• Glucose and fructosamine — how well diabetes is controlled; poor control despite large amounts of insulin is the usual reason to test IGF-1.
• Total T4 — hyperthyroidism is one of the other causes of insulin resistance that the vet rules out; in puppies, thyroid function affects IGF-1 itself.
• Imaging of the pituitary — CT of the pituitary is described as the most definitive test for feline acromegaly.

Units and conversion

IGF-1 is usually reported in ng/mL; some labs use nmol/L. One nmol/L equals 7.649 ng/mL, so 1,000 ng/mL is about 131 nmol/L. In the app each row keeps the unit printed on the form; the chart converts to ng/mL so forms from different labs line up.

Reference range

The reference range is on your lab’s form, and for a diabetic cat labs often add an interpretation comment. One university endocrinology laboratory and the AAFP diabetes toolkit describe an IGF-1 above 1,000 ng/mL in a diabetic cat as supporting acromegaly; values of about 800–1,000 ng/mL are equivocal in some studies, and some extend the grey zone down to 500 ng/mL when signs fit. Because IGF-1 can be falsely low early on, the toolkit advises testing after about 6 weeks on insulin, and the laboratory notes that an equivocal result with ongoing suspicion may be repeated in 2–6 months.

Cats and dogs

Sources

1. Michigan State University Veterinary Diagnostic Laboratory — Growth Hormone and IGF-1 in Cats
2. Michigan State University Veterinary Diagnostic Laboratory — Growth Hormone and IGF-1 in Dogs
3. Greco D. S. Acromegaly in Cats. Merck Veterinary Manual, professional version, full review September 2024
4. AAFP Diabetes Educational Toolkit — Troubleshooting (catvets.com)
5. Merck Veterinary Manual — Juvenile-Onset Panhypopituitarism in Dogs (professional version)