Free T4 represents the unbound portion of thyroxine, (<1%) which is metabolically active. It therefore provides a more accurate status of thyroid function than other thyroid tests. It is much less likely to be influenced by other factors such as drugs and non-thyroidal illness (NTI)2. The free T4 test is significantly more sensitive in detecting hyperthyroidism in mildly hyperthyroid cats than either total T4 or T3 measurements. However, the free T4 test occasionally has a false positive test result, so hyperthyroidism should not be diagnosed solely on the basis of free T4 determination4.
The IMMULITE veterinary FT4 assay accurately differentiates dogs and cats with thyroid disease, providing superior precision and offering a faster turnaround time than the traditional equilibrium dialysis method (ED).
In clinical studies, the IMMULITE FT4 assay accurately differentiated 80% of dogs as hypothyroid and 97% of dogs as euthyroid for an accuracy of 89% in dogs with clinical signs of hypothyroidism4. In addition, the IMMULITE FT4 assay accurately identified 87% of cats as hyperthyroid and 100% of cats as euthyroid for an accuracy of 89% in cats with clinical signs of hyperthyroidism5.
Veterinary T4 and TSH
In addition to FT4, the total T4 and TSH assays are considered useful screening tests for canine hypothyroidism. Total T4 may cause misdiagnosis of thyroid disorders, since it is affected by NTI, and certain medications. Therefore, relying on either test alone is not recommended. Although TSH is typically used as a screening tool, in a clinical study using the IMMULITE canine TSH assay, results indicated that the measurement of canine TSH had an excellent specificity (100%) and was a valuable tool in confirming canine hypothyroidism. It was not, however, recommended for excluding the disease6.
Veterinary TLI (trypsin-like immunoreactivity)
Exocrine pancreatic insufficiency (EPI) is a syndrome caused by insufficient synthesis and secretion of digestive enzymes by the exocrine portion of the pancreas. It is much more common in dogs than cats7. EPI is an uncommon cause of chronic diarrhea in cats; however, in the past it has been under diagnosed due to the lack of specific clinical and laboratory findings. Diagnostic accuracy has now been facilitated by the TLI test.
Serum TLI concentration is the diagnostic test of choice for EPI in both dogs and cats. Assays for TLI measure trypsinogen circulating in the vascular space. In healthy individuals, only a small amount of trypsinogen is present in serum. However, in dogs and cats with EPI, serum TLI decreases significantly and may even be undetectable7.
1. 2011-2012 APPA National Pet Owners Survey
2. Veterinary Practice News. www.veterinarypracticenews.com posted April 12, 2011 by W. Jean Dodds, DVM
3. Brochure on Hyperthyroidism in Cats. Prepared b the American Association of Feline Practitioners and the Cornell Feline Health Center, Cornell University, College of Veterinary Medicine 2007
4. Veterinary Clinical pathology clerkship program. www.vet.uga.edu Feline Hyperthyroidism. By Jeff S. Stortz, DVM; Kenneth S. Latimer, DVM, PhD; Heather L. Tarpley, DVM; Bruce E. LeRoy, DVM, PhD, Perry J. Bain, DVM, PhD, T. Michelle Wall, DVM, DACVIM
5. IDEXX Laboratories. Diagnostic Update. March 2011.IDEXX Reference laboratories new Free T4 Test
6. Boretti, FS, Reusch C.E.- Diagnostic specificity of canine thyrotropin in the diagnosis of Hypothyroidism in dogs. EJCAP 2006,16:185-189
7. The Merck Veterinary Manual. © 2011; Merck Sharp & Dohme Corp., a subsidiary of Merck & Co., Inc