Key points
• Older cats with hyperthyroidism frequently have concurrent renal disease or UTI.
• Concurrent diseases can suppress total thyroid hormone measurements in cats and confuse the diagnosis of hyperthyroidism.
• GFR is increased by the hyperthyroid state and declines over the first month after treatment.
• Hyperthyroid cats with concurrent renal disease require a trial treatment period with reversible anti-thyroid
medications in order to determine the likely effects of definitive treatment on renal function.
• Systemic hypertension and proteinuria may resolve after treatment of hyperthyroidism.
• Hyperthyroid cats with concurrent chronic kidney disease, persistent systemic hypertension or proteinuria require
specific management of these disorders in addition to management of hyperthyroidism.
• UTI in hyperthyroid cats should be managed as complicated UTI.
Polyuria/polydipsia in cats
Although cats may not demonstrate polyuria/polydipsia in as dramatic a fashion as dogs, a change in water intake or increase
in urine in the litter box warrants further investigation. Owners will readily notice a change in habits if a cat begins to
empty the water bowl or seeks water from other sources, such as a dog bowl, sink, faucet or shower. Polyuria is confirmed
by documenting dilute urine in several random samples. Recall that normal cats usually concentrate urine to a specific gravity
much greater than 1.035.
The most common causes of PU/PD in cats include chronic kidney disease, hyperthyroidism and diabetes mellitus, all diseases
of geriatric cats that can co-exist in an individual cat. In hyperthyroidism, it is thought that changes in medullary blood
flow alter distal concentrating mechanisms. Psychogenic polydipsia may result from thyrotoxicosis in some cats as well. A
data base including biochemical data, thyroid hormone measurement and urinalysis is usually adequate for diagnosis of pu/pd
in cats.
Hyperthyroidism and renal function
High circulating thyroid hormone directly impacts renal blood flow and the hemodynamic forces favoring glomerular filtration.
Filtration forces and tubular functions are increased. In the short term, hyperfiltration will preserve total GFR even if
the kidneys have lost functional nephrons. In cats with marginal renal function, treatment of hyperthyroidism may remove
these positive forces and reveal ("unmask") the biochemical or clinical effects of the underlying renal dysfunction. On the
other hand, sustained glomerular hypertension is a known mechanism perpetuating renal disease in many species. Systemic hypertension
associated with untreated hyperthyroidism also is damaging to renal and other capillary beds and is a risk factor for progressive
dysfunction.
Concurrent hyperthyroidism and chronic kidney disease
Chronic kidney disease is usually discovered by finding alterations in renal structure, urine concentrating ability, biochemical
indicators (BUN, creatinine) or persistent proteinuria. In a hyperthyroid cat, however, urine specific gravity is a unreliable
indicator of renal function; BUN and creatinine can be affected by dehydration (or conversely, decreased by muscle wasting
and malnutrition). Likewise the stage or degree of renal disease cannot be determined by the level of azotemia in a CKD cat
with concurrent hyperthyroidism. Other clues become important in determining the relative contribution of renal disease to
the cat's illness: patient history, prior laboratory findings, kidney size or architecture. Measures of GFR can provide more
objective information, although GFR is affected by thyroid hormone levels. GFR measurements lower than 2.25 ml/min/kg as measured
by DTPA scintigraphy are associated with worsening of renal parameters after radioiodine treatment in hyperthyroid cats.
Plasma clearance methods are becoming more practical for veterinary practice and can be used to provide some objective measure
of renal function in questionable cases. GFR can be expected to decline to some degree in all cats after definitive treatment
of hyperthyroidism (radioiodine, surgical thyroidectomy), but the decline appears to stabilize at about 4 weeks post-treatment.
Hyperthyroidism, hypertension and proteinuria
Because older cats often suffer from hypertensive disorders (hyperthyroidism, diabetes mellitus, chronic kidney disease, or
idiopathic hypertension), serial monitoring of blood pressure, fundic examination and urine protein:creatinine ratio should
be part of the management strategy. UPC serves as a key marker for the risk of, and progression of CKD in cats. Specific
management of proteinuria may be required in cats with persistent, significant proteinuria (UPC > 0.4) despite control of
underlying hyperthyroidism or hypertension.
