The Pulse Of Veterinary Medicine
Vet-To-Vet Conversation With IDEXX
Hyperthyroidism is a disease seen primarily in older cats, in which kidney disease is also common.
The clinical challenge is that hyperthyroidism can mask the presence of kidney disease,1 and there has not been a reliable routine diagnostic test that can assess kidney function in cats affected by hyperthyroidism—until now.
Creatinine, being a by-product of muscle, is underproduced in feline hyperthyroidism as a result of muscle loss and becomes a poor indicator of kidney function. Creatinine is also lowered by the hyperfiltration associated with the increased metabolic state.1
The IDEXX SDMA Test is not impacted by weight loss and muscle mass2 and appears to be only slightly blunted by hyperfiltration,3 making it a much more reliable marker of kidney function in hyperthyroid cats.
SDMA is not impacted as muscle mass decreases. Creatinine decreases as muscle mass decreases.
Data research findings
When 1,959 cats were evaluated, creatinine levels dropped, whereas the SDMA* concentrations remained relatively unchanged. Thus SDMA was able to identify significantly more cats with poor kidney function. When provided, the average body weight had significantly declined in this population of cats.3
Cats with hyperthyroidism experience significant weight loss.
Creatinine decreases with loss of muscle mass and high total T4 values in hyperthyroid cats, making it unreliable in detecting underlying kidney disease.
SDMA is not impacted by loss of muscle mass and is only marginally decreased in hyperthyroid cats with high serum total T4 values, making it a more reliable indicator of underlying kidney disease.
SDMA is a more reliable indicator of kidney function in hyperthyroid cats than creatinine. To demonstrate that SDMA remains relatively stable in the face of both muscle loss and hyperfiltration, a large retrospective data analysis3 was performed comparing creatinine and SDMA results in cats older than 5 years of age. It also compared the number of cats with kidney disease identified by creatinine versus the number of cats identified by SDMA as having kidney disease.
The occurrence of kidney disease was evaluated in an age-matched general population of cats and compared to a hyperthyroid cat population.
In the general cat population: Creatinine was found to be increased above the reference interval in 14% of the cats; SDMA was increased in 27% (figure 1).
These results align with numerous studies that have shown that SDMA can detect kidney disease when creatinine is normal because SDMA is highly correlated to kidney function, not impacted by muscle mass, not impacted by diet or gender, and more sensitive at identifying kidney disease earlier than creatinine. These findings suggest that by using the IDEXX SDMA Test, veterinarians have the opportunity to diagnose kidney disease 2 times more in feline patients.
In the hyperthyroid cat population: Creatinine results above the reference interval decreased dramatically to 3.5% compared to the 14% observed in the age-matched general cat population; whereas, the proportion of cats with increased SDMA above the reference interval remained similar to the general cat population at 20.6% (figure 2).
This robust retrospective data analysis demonstrated that there is a profound effect on creatinine results in hyperthyroid cats.
The IDEXX SDMA Test reliably identified 6 times more hyperthyroid cats with kidney disease than creatinine. Creatinine missed 82%† of hyperthyroid cats that the IDEXX SDMA Test identified as having kidney disease.
†Kidney disease was identified in 72 hyperthyroid cats out of the total general feline population (n = 2,000), or 3.5%, using creatinine alone. However, kidney disease was identified in 412 hyperthyroid cats out of the total general feline population, or 20.6%, by adding the IDEXX SDMA Test. This means that using creatinine alone missed 340 hyperthyroid cats with kidney disease, or 82%.
1. Williams T. Chronic kidney disease in cats with hyperthyroidism. Clin Brief. Sept 2015:10–12.
2. Hall JA, Yerramilli M, Obare E, Yerramilli M, Yu S, Jewell DE. Comparison of serum concentrations of symmetric dimethylarginine and creatinine as kidney function biomarkers in healthy geriatric cats fed reduced protein foods enriched with fish oil, L-carnitine, and medium-chain triglycerides. Vet J. 2014;202(3):588–596.
3. Data on file at IDEXX Laboratories, Inc. Westbrook, Maine USA.