Hyperthyroidism is one of the most commonly diagnosed diseases of the older cat. Geriatric cats with hyperthyroidism may also
have concurrent chronic kidney disease (CKD). Systemic hypertension, proteinuria, and urinary tract infection (UTI) can be
consequences of either hyperthyroidism or CKD. Hyperthyroidism can increase glomerular filtration rate (GFR) in cats with
CKD which can attenuate or resolve mild to moderate azotemia. In addition, serum creatinine may be decreased in cats with
weight loss and decreased muscle mass. In both cases, reductions in BUN and serum creatinine concentrations make it more difficult
to detect concurrent CKD. Conversely, the CKD may depress thyroid hormone concentrations (euthyroid sick syndrome) making
it more difficult to diagnose hyperthyroidism. Initial treatment of cats with suspected or confirmed CKD should ideally be
accomplished with a reversible anti-thyroid medication in order to assess any adverse effects on renal function. Systolic
blood pressure and urine protein creatinine ratio (UP/C) should be evaluated prior to and after treatment. Urine cultures
should be obtained as part of the workup of both hyperthyroidism and CKD. In either case, a concurrent UTI should be managed
as a complicated UTI with long-term antibiotic treatment based on culture and sensitivity results.
Clinical Signs/Physical Examination
Classic clinical signs of hyperthyroidism include weight loss, polyuria/polydipsia (PU/PD), and polyphagia in an older cat.
Fewer than 5% of cats with hyperthyroidism are less than 8 years of age; the average age at diagnosis is 12-13 years. A thyroid
enlargement (thyroid slip) can often be palpated in hyperthyroid cats, although some euthyroid cats will also have enlargement
of one or both glands. In cats with concurrent CKD, kidneys may be small and/or irregular. Approximately 50% of cats with
hyperthyroidism will exhibit PU/PD. A primary polyuria may occur as a result of thyrotoxicosis increasing cardiac output and
GFR as well as increased renal medullary blood flow which has the potential to decrease renal medullary hypertonicity and
urine concentrating ability. Some cats with hyperthyroidism may also have a primary polydipsia secondary to the effects of
high thyroid hormone concentrations on the thirst center. Regardless of the mechanism, decreased urine specific gravity makes
interpretation of azotemia problematic (is it pre-renal azotemia superimposed on decreased concentrating ability or renal
azotemia?). Systemic hypertension is another common finding in hyperthyroid cats. High blood pressure may be caused by increased
cardiac output, sympathetic tone, and arteriolar resistance and if sustained, can lead to intraglomerular hypertension, glomerular
sclerosis, and proteinuria. No matter what the underlying cause, hypertension can damage the eyes, brain, heart, and kidney
of affected cats. For example, tachycardia murmurs, and gallop rhythms may be associated with hypertrophic cardiomyopathy.
Similarly, whether proteinuria arises from hypertension or CKD, progressive renal disease is a potential consequence.
Increased practitioner awareness of hyperthyroidism, an increasing population of geriatric cats, and increased diagnostic
testing of older cats (wellness exams) has resulted in earlier diagnosis of hyperthyroidism in many cases. Clinical signs
in these cases may be more subtle compared with an advanced case of hyperthyroidism. With earlier diagnosis, weight loss may
be present but emaciation will be less likely and body condition scores will be higher. Similarly, PU/PD is less likely to
be observed by owners and appetite and activity levels may be only slightly increased in cats with early hyperthyroidism.
Clinicopathologic findings associated with hyperthyroidism may include a slight erythrocytosis; perhaps secondary to increased
tissue oxygen consumption. Serum ALT is increased in approximately 75-90% of cats with hyperthyroidism and is thought to be
associated with malnutrition, hepatic hypoxia, and/or toxic effects for thyroid hormone on hepatocytes. Azotemia is observed
in approximately 25% of hyperthyroid cats and may be due to dehydration, increased protein turnover (BUN), and/or CKD. Concurrently,
urine specific gravity is often decreased as discussed previously.