IDEXX Total T4 Testing Guide
Canine Hypothyroidism
0.5–10.0 µg/dL
(6.4–128.7 nmol/L)
Catalyst Total T4 Results
| Low | <1. 0 µg/dL | (<13.0 nmol/L) |
| Low Norma | 1.0–2.0 µg/dL | (13.0–26.0 nmol/L) |
| Normal | 1.0–4.0 µg/dL | (13.0–51.0 nmol/L) |
| High | >4.0 µg/dL | (>51.0 nmol/L) |
| Therapeutic | 2.1–5.4 µg/dL | (27.0–69.0 nmol/L) |
Canine screening
- Dogs with low total T4 (T4) and evidence of nonthyroidal illness (NTI) should have the NTI addressed.
- Dogs with T4 results in the low normal range may be hypothyroid.
In dogs with low or low normal T4 results and with consistent clinical signs, consider one or more of the following to aid in confirming hypothyroidism:
- Free T4 (fT4)
- Endogenous thyroid-stimulating hormone (TSH)
- Possible thyroglobulin autoantibodies (TgAA)
Hypothyroidism therapeutic monitoring
For dogs on thyroid supplement, acceptable 4–6-hour post-pill T4 values will generally fall in the upper end of the reference interval or slightly above.
Algorithm
CBC = Complete blood count
Note: 1 µg/dL is equal to 12.87 nmol/L. A result that falls within the low normal range of the assay should be considered ambiguous
Feline Hyperthyroidism Dynamic range
0.5–20.0 µg/dL
(6.4–257.4 nmol/L)
Catalyst Total T4 Results
| Subnormal | <0.8 µg/dL | (<10.0 nmol/L) |
| Normal | 0.8–4.7 µg/dL | (10.0–60.0 nmol/L |
| Gray zone in old or symptomatic cats | 2.3–4.7 µg/dL | (30.0–60.0 nmol/L) |
| Consistent with hyperthyroidism | >4.7 7 µg/dL | (>60.0 nmol/L) |
Feline screening
Cats with consistent clinical signs and total T4 (T4) values in the borderline high range (gray zone) may have early hyperthyroidism with a concurrent nonthyroidal illness (NTI). In these cases, consider the following to aid in confirming the diagnosis:
- Free T4 (fT4)
- T3 suppression test
- Radionuclide thyroid imaging
Hyperthyroidism therapeutic monitoring
Following treatment with methimazole (or similar), T4 values will generally fall within the lower to mid portion of the reference interval.
Algorithm
If strong suspicion of hyperthyroidism still exists, consider retesting in 4–6 weeks or a technetium scan.
CBC = Complete blood count
Note: 1 µg/dL is equal to 12.87 nmol/L. A result that falls within the gray zone of the assay should be considered ambiguous.
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Documents / Resources
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IDEXX Total T4 Testing Guide [pdf] User Guide Total T4 Testing Guide, T4 Testing Guide, Testing Guide |
