When T4 and Free T4 Don’t Match
Julie Piccione, DVM, MS, DACVP, and Malcolm Delovio, PhD
A two-year-old mixed breed dog was presented to their veterinarian for an inability to lose weight and mild hair loss. Blood was collected and serum was submitted to the Texas A&M Veterinary Medical Diagnostic Laboratory (TVMDL) for the Canine Basic Thyroid Panel. Results are reported below.
| Test | Result | Flag | Reference Interval | Unit | Method |
| T4 | 0.61 | L | 1.51 – 3.11 | ug/dL | Chemiluminescence |
| Free T4 | 45.20 | H | 9.0 – 33.0 | pmol/L | Chemiluminescence |
| TSH | 1.64 | H | 0.04 – 0.40 | ng/mL | Chemiluminescence |
The low T4, substantially high TSH, and the clinical history were compatible with canine hypothyroidism. However, the discrepancy between T4 and Free T4 in this case was interesting to the submitting veterinarian, even though it is commonly observed in the diagnostic laboratory. The autoantibodies that cause immune-mediated lymphocytic thyroiditis in dogs can be measured as Free T4 with some chemiluminescence assays.
A Thyroglobulin Autoantibody (TgAA) test was ordered to confirm the presence of interfering antibodies. The TgAA test was positive: confirming the presence of antibodies, interference in the Free T4 assay, and ultimately confirming the diagnosis of immune-mediated hypothyroidism in this case.
This case provides one example of a discrepancy between T4 and Free T4 values. T4 refers to total T4, which includes T4 bound to proteins and unbound T4. Free T4 is the proportion of T4 that is not protein bound. This is considered the biologically active form of T4 as it can freely enter most cells.
While total T4 may also be falsely increased when autoantibodies are present, in our laboratory Free T4 is more commonly affected. It is important to note that the cross reaction will only be observed in Free T4 measured with chemiluminescence methods and not in Free T4 measured by equilibrium dialysis. However, the expense of measuring Free T4 by equilibrium dialysis is often considered in the diagnostic plan. Measuring Free T4 by equilibrium dialysis involves dialyzing the patient’s sample through a membrane for 17 hours. The membrane has small pores that protein-bound T4 and antibodies cannot cross; therefore, they are not measured in the final step of this assay.
For more information on thyroid testing at TVMDL, please visit tvmdl.tamu.edu or contact us at 979-845-3414.