| Material | serum or heparin plasma 0.5 ml or 0.4 ml in small mammals |
| Method | CLIA (chemiluminescence assay) |
| Species list | dog, cat, guinea pig, horse, cattle, camelids, others on request |
| Duration | Reporting of results on the day the sample is received (Mon–Sat) |
| Remark | - Dog: Most commonly used to diagnose hypothyroidism. Cave: A low T4 concentration is not necessarily associated with hypothyroidism. The assessment should be made together with TSH. rT3 can provide further information to help distinguish between non-thyroid causes of reduced T4 concentration. If the findings are inconclusive, a TRH Stimulation Test (see Endocrinology/Funktion Tests) can be performed.
- Therapy monitoring for hypothyroidism in dogs: 4–6 hours after administration of thyroxine; therapy monitoring every 2–4 weeks until clinical stabilisation is achieved, then every 3–6 months. Target: middle to upper reference range.
- Cat: Most commonly used to diagnose hyperthyroidism. Elevated serum T4 concentrations indicate hyperthyroidism. T4 in the (upper) reference range does not necessarily rule out hyperthyroidism. If in doubt, a combination of determining T4, fT4 and TSH and repeat measurements can help.
- Therapy monitoring for hyperthyroidism in cats: Therapy monitoring every 2–4 weeks until good control is achieved, then every 3–6 months. Target: medium reference range. T4 concentrations that are too low should be avoided.
- Dog, cat: T4 can be determined by LCMS measurement without interference from metabolites or antibodies. This analysis can be requested via the Thyroid Mass Spectrometry profile.
- Small mammals: T4 is suitable for diagnosing hyperthyroidism. Hypothyroidism has not yet been described in small mammals.
- Horse: For very rare cases of hypothyroidism, determination of T4 and T3 followed by a TRH Stimulation Test is recommended (see Endocrinology/Funktion Tests).
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