Thyroxine (T4) Testing Guidelines by Species
Dog
Used in the diagnosis of hypothyroidism. As a standalone value, T4 has limited diagnostic significance. It should always be interpreted alongside free T4 (fT4) and TSH, or a functional test should be performed.
Cat
Used in the diagnosis of hyperthyroidism — the most common hormonal disorder in older cats, often associated with tachycardia, chronic diarrhoea, and cachexia. T4 is usually sufficient as a single parameter. In borderline cases, TSH and fT4 may be assessed additionally.
Therapy Monitoring (Dog/Cat)
For dogs, blood sampling should be performed 4–6 hours after thyroxine administration. Therapy monitoring is recommended 2–4 weeks after treatment initiation.
Birds and Reptiles
T4 values are often physiologically very low — sometimes below the detection limit of standard test systems. In such cases, fT4 determination is recommended.
Horse
If hypothyroidism is suspected (rare), both T4 and T3 should be measured, followed by a TRH stimulation test for confirmation.
In birds and reptiles we offer the same test in LC-MS/MS (Liquid Chromatography–Tandem Mass Spectrometry) method see Thyroxin T4 in birds and reptiles
