5. Perform clinical diagnostic testing
The time to evaluate the patient for an endocrine disease will depend on the patient's initial history and physical examination
findings. In many cases, the veterinarian will address the secondary skin infections that may be causing pruritus at the initial
examination and then re-evaluate the patient 21 days after such therapy is finished. If the patient's pruritus and lesions
have resolved, but clinical signs are still present (suggesting an endocrinopathy), then screening for hypothyroidism and
Cushing's disease is indicated.
Initial screening tests for suspected endocrinopathy include a complete blood count, serum chemistry profile, urinalysis and
thyroid screening, including thyroxine (T4), free T4 by equilibrium dialysis and thyroid-stimulating hormone (TSH) measurement. The following changes can be detected in patients
with an endocrinopathy:
Marked serum lipemia. In many patients, their ability to metabolize fat is affected by endocrine disease. In some cases, patients will present
with a marked serum lipemia, evident once the blood tube is spun down. Lipemia can occur if a patient's blood is drawn immediately
after eating or with other medical conditions. However, if lipemia is noted, the staff should bring it to the veterinarian's
Mild changes in blood studies. By themselves, mild changes are not indicative of endocrine disease, but they should be reviewed in conjunction with the entire
medical database. Hypothyroid patients can present with normocytic, normochromic, nonregenerative anemia. Patients with Cushing's
disease often have neutrophilia, monocytosis, lymphopenia and eosinopenia.
Changes in clinical chemistry. Just as the complete blood count results can reveal changes indicating endocrine disease, the serum chemistry profile findings
may show changes, raising the level of suspicion that an endocrinopathy exists. For example, in hypothyroidism, increases
in serum cholesterol and lipid concentrations can be observed secondary to improper metabolism of dietary fats. Thyroid hormone
is necessary for all facets of lipid metabolism, but it is especially important in the mobilization and degradation of lipids.
Liver enzyme activities, such as alkaline phosphatase, aspartate aminotransferase and alanine aminotransferase, may be increased
secondary to fatty liver changes in the patient.
Meanwhile, increased alkaline phosphatase is the most common biochemical abnormality in dogs with hyperadrenocorticism, with
increased activities being present in 90 percent to 95 percent of these patients. Also an increased cholesterol concentration
is seen in 90 percent of dogs with this disorder due to glucocorticoid stimulation of lipolysis.
Glucocorticoids will increase blood glucose and decrease peripheral utilization. Therefore, patients with hyperadrenocorticism
can have slight to moderate increases in blood glucose concentration. Lastly, increased alanine aminotransferase activity
is likely due to hepatocytes swollen with glycogen accumulation.
Urinalysis. Patients with hyperadrenocorticism often have dilute urine (urine specific gravity < 1.015). A urine sample also can be used
to screen for diabetes mellitus, proteinuria and urinary tract infection, all of which are common sequelae of hyperadrenocorticism.
Thyroid testing. No one thyroid test is 100 percent diagnostic for hypothyroidism. Basal measurements of total T4, free T4, free T4 by equilibrium dialysis (which is more accurate than free T4) and TSH are the mainstays of testing. Typically, hypothyroid patients have decreased total T4 and free T4 concentrations and increased TSH concentrations. Many factors, such as concurrent illness and drug therapy, will influence
the resting total T4 concentration. Even the values set by the laboratory will determine if dogs are hypothyroid; if a reference laboratory sets
the bottom of the range too high, normal dogs will appear to be hypothyroid.
Patients with other underlying or concurrent illnesses may be incorrectly determined to be hypothyroid due to a condition
called euthyroid sick syndrome. In this syndrome, chronic disease and drug therapy can decrease serum thyroid hormone concentrations,
decrease thyroid gland responsiveness to TSH and decrease pituitary secretion of TSH. Once these patients are treated for
the primary disease, their thyroid function returns to normal. Disorders known to cause euthyroid sick syndrome include chronic
infection (especially deep pyoderma) and hyperadrenocorticism. This can result in a confusing and challenging situation, and
the veterinarian will need to discern if the hypothyroidism caused the deep pyoderma, or if the deep pyoderma caused the euthyroid
sick syndrome. Drugs that may decrease thyroid hormone concentrations include glucocorticoids, phenobarbital, nonsteroidal
anti-inflammatory drugs, penicillin antibiotics, imidazole antifungals, furosemide, diazepam, antineoplastic agents and phenothiazines.