Diagnosing Hyperthyroidism in Cats: Routine Testing Procedures

A variety of standard testing procedures are recommended in the workup of cats suspected of suffering from hyperthyroidism. These tests in include a complete blood count, serum chemistry analysis, and urinalysis.

Often this database simply lends support to the diagnosis, but these screening tests should always be done. They are essential in determining if a hyperthyroid cat also has concurrent medical problems (e.g., kidney, heart, or liver disease). These conditions may affect which form of treatment is recommended, as well as providing a more accurate prognosis.

Many hyperthyroid cats will develop secondary heart issues, including a rapid heart rate, heart murmurs, heart enlargement, or abnormal heart rhythms. Therefore heart procedures, such as a chest radiograph (x-ray), electrocardiogram (ECG), or echocardiogram, may also be important in the workup of a cat with suspected hyperthyroidism.

Routine Bloods Testing

Complete blood count
Hematological findings are usually non-specific and mostly not clinically important.

• An increased number of red blood cells (erythrocytosis with increased packed cell volume (PCV),  red blood cell (RBC) count and hemoglobin concentration) are relatively common. These changes reflect thyroid hormone stimulation of the bone marrow.
• A high white blood cell (WBC) count associated with a stress leukogram is common. With this pattern, which is associated with clinical states of stress, we see high numbers of neutrophils accompanied by low numbers of lymphocytes and eosinophils.
• Anemia, when present, is almost never caused by the hyperthyroid state and a search for another cause should be undertaken.

Serum chemistry panel
Serum biochemistry abnormalities are common in cats with hyperthyroidism (see Table).

• Mild to marked increases in the serum activities of many liver enzymes, including alanine aminotransferase (ALT) and alkaline phosphatase (ALP) are the most common and striking biochemical abnormalities of feline hyperthyroidism. These liver enzymes changes and T4 concentrations are related, with liver enzyme abnormalities being more common in cats with severe hyperthyroidism. These high liver enzymes return to normal upon successful treatment of hyperthyroidism.
• Before treatment, mild to moderate increases in serum concentrations of the kidney tests, urea nitrogen and creatinine, may be found in just over 20% of hyperthyroid cats. Such a prevalence of concurrent renal dysfunction or chronic kidney disease (CKD) is not unexpected in a group of older or aged cats. 
• These abnormalities, particularly the high urea concentration, may be exacerbated by the increased protein intake and protein catabolism of hyperthyroidism. On the other hand, in hyperthyroid cats without concurrent CKD or azotemia, circulating creatinine concentrations are lowered, which may be related in part to a loss of muscle mass.
• However, this lowering of serum creatinine (and urea in some cats) is primarily the result of the increase in glomerular filtration rate (GFR) that occurs in hyperthyroid cats. These effects have implications in assessing the presence of primary renal dysfunction in hyperthyroid cats (common to see “masked” kidney disease in untreated hyperthyroid cats that only becomes apparent after treatment).

A number of other changes can develop in hyperthyroid cats but are either clinically insignificant or rarely encountered.

• A high serum phosphate concentration without evidence of kidney disease occurs in approximately 35-45% of cats.
• Blood glucose (sugar) concentrations may be mildly increased presumably reflecting a stress response. In cases with pre-existing diabetes mellitus, accelerated insulin catabolism increases requirements for exogenous insulin.
• A low serum potassium (hypokalemia) has occasionally been associated with hyperthyroidism and should be suspected in any cat with evidence of severe muscle weakness.

Complete urinalysis
Urinalysis is generally unremarkable but is useful in differentiating other diseases with similar clinical signs such as diabetes mellitus.

• The urine specific gravity is variable, but concurrent (masked) renal disease should be considered in all cats that have dilute urine (urine specific gravity, less than1.040). Cats with concurrent CKD can occasionally have more concentrated urine values (specific gravity, greater than 1.040), but most cats have less concentrated urine.
• Mild increases in urine protein excretion (proteinuria) is common. Proteinuria found on routine urinalysis should be confirmed by measuring the urine protein-to-creatinine ratio (UPC ratio).
• Normal cats have a UPC ratio of less than 0.2, where many hyperthyroid cats have borderline high (0.2-0.4) or overt proteinuria (greater than 0.4). Such proteinuria resolves upon successful treatment of hyperthyroidism.
• Urinary tract infections occur in approximately 10% of hyperthyroid cats. As a consequence, urine culture is indicated in the workup of cats with hyperthyroid cats.

Table 1: Routine test abnormalities associated with hyperthyroidism in cats

Complete blood count

• High red cell count
• High white blood cell count
• Low numbers of lymphcytes
• Low numbers of eosinophils
• Low red cell count (anemia) — very rare

Serum chemistry profile

• High liver tests (ALT and ALP)
• High kidney tests
• High blood sugar (glucose)
• High phosphorus

Complete urinalysis

• Dilute or concentrated urine
• High urine protein excretion
• Infection (white blood cells, bacteria)

Heart Testing Procedures

Chest Radiographs (x-rays)

• On thoracic radiography, mild to severe heart enlargement (cardiomegaly) is evident in about half of hyperthyroid cats. In the vast majority of cats, this cardiomegaly is reversible with correction of the hyperthyroid state.
• With severe hyperthyroidism, cats may develop congestive heart failure. In these cats, chest x-rays may show evidence of a buildup of fluid around the lungs (pleural effusion) or within the lungs themselves  (pulmonary edema).

Echocardiogram

• The most common echocardiographic findings in hyperthyroid cats include left ventricular hypertrophy (thickening), left atrial and ventricular dilation, and interventricular septum hypertrophy. 
• However, most of these changes are subtle and are of little clinical relevance.
• Increased cardiac contractility is common and invariably normalizes upon successful treatment of the hyperthyroidism.

Echocardiogram

• A rapid heart rate (sinus tachycardia) develops in over half of hyperthyroid cats. 
• Other changes seen on the electrocardiogram (ECG) may include increased R-wave amplitude, prolonged QRS duration, shortened Q-T interval, and intraventricular conduction disturbances. 
•  In severe hyperthyroidism, a variety of atrial and ventricular arrhythmias can develop.

Confirming the Diagnosis

Confirming the diagnosis of hyperthyroidism is usually straightforward and is done by performing specific thyroid function tests.  I’ll cover this topic in my next post.

However, screening serum biochemistry and hematology remain extremely important to assess concurrent conditions that may impact the management if hyperthyroidism is diagnosed, as well as provide baseline values for parameters that can be affected by treatment (such as the kidney function tests).

References:

1. Bond BR, Fox PR, Peterson ME,  et al.  Echocardiographic findings in 103 cats with hyperthyroidism. Journal of the American Veterinary Medical Association 1988; 192:1546-1549
2. Broussard JD, Peterson ME, Fox PR. Changes in clinical and laboratory findings in cats with hyperthyroidism from 1983 to 1993. Journal of the American Veterinary Medical Association 1995; 206:302-305.
3. DiBartola SP, Brown SA. The kidney and hyperthyroidism. In: Kirk’s Current Veterinary Therapy XIII, ed. JD Bonagura,  WB Saunders, Philadelphia, 2002; pp. 337-339.
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8. Peterson ME, Keene B, Ferguson DC,  et al. Electrocardiographic findings in 45 cats with hyperthyroidism. Journal of the American Veterinary Medical Association 1982;180:934-937.
9. Peterson ME, Kintzer PP, Cavanagh PG, et al.  Feline hyperthyroidism: pretreatment clinical and laboratory evaluation of 131 cases. Journal of the American Veterinary Medical Association 1981;183: 103-110.
10. Reusch CE, Tomsa K. Serum fructosamine concentration in cats with overt hyperthyroidism. Journal of the American Veterinary Medical Association 1999;215:1297-1300.
11. Thoday KL, Mooney CT.  Historical, clinical and laboratory features of 126 hyperthyroid cats. Veterinary Record 1992;131:257-264.