C-4
Interpreting Test Results
The recommendations below are intended to provide general guidance only. As with any diagnosis,
you should use clinical discretion with each patient based on a complete evaluation of the patient,
including history, physical examination, and complete laboratory profile.
Species Results
U.S. Units (µg/dL) S.I. Units (nmol/L)
Canine
†
<1.0 Low <13 Low
1.0–2.0 Low Normal 13–26 Low Normal
1.0–4.0 Normal 13–51 Normal
>4.0 High >51 High
2.1–5.4 Therapeutic 27–69 Therapeutic
Feline
‡
<0.8 Subnormal <10 Subnormal
0.8–4.7 Normal 10–60 Normal
2.3–4.7 Gray zone in old or
symptomatic cats
30–60 Gray zone in old or
symptomatic cats
>4.7 Consistent with
hyperthyroidism
>60 Consistent with
hyperthyroidism
Equine <1.0 Low <13 Low
1.0–3.8 Normal 13–49 Normal
>3.8 High >49 High
†
Canine Screening
• Dogs with a low total T
4
(T
4
) and evidence of nonthyroidal illness (NTI) should have their NTI addressed.
• Dogs with T
4
results in the low normal range may be hypothyroid.
• In dogs with low or low normal T
4
results and with consistent clinical signs, consider free T
4
(fT
4
) and
endogenous thyroid-stimulating hormone (TSH) and possibly thyroglobulin autoantibodies (TgAA) to aid in
confirming hypothyroidism.
Hypothyroidism Therapeutic Monitoring
For dogs on thyroid supplement, acceptable 4–6-hour post-pill T
4
values will generally fall in the upper end of
the reference range or slightly above.
‡
Feline Screening
Cats with consistent clinical signs and total T
4
(T
4
) values in the borderline high range (gray zone) may have
early hyperthyroidism or a concurrent nonthyroidal illness (NTI). In these cases, consider a free T
4
(fT
4
), a T
3
suppression test or radionuclide thyroid imaging to aid in confirming the diagnosis.
Hyperthyroidism Therapeutic Monitoring
Following treatment with methimazole, T
4
values will generally fall within the lower to midportion of the
reference range.
SNAP Test Procedures
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