Species: Canine and feline

Specimen: 1 mL serum

Container: Plain (red top) tube

Collection protocol: Standard venepuncture

Special handling/shipping requirements:

Send samples to the laboratory within 12-24 hours of collection. 

General information about the disease:

Free-T4 is a measure of metabolically active, unbound thyroid hormone and makes up approximately 1% of the circulating thyroid hormone concentration. Free-T4 (FT4) is that portion of the thyroid hormone that enters into cells to perform metabolic functions and exerts a negative feedback on pituitary production of TSH. Currently FT4 in dogs and cats is measured at Awanui Veterinary using a modified equilibrium dialysis method. This entails a preliminary dialysis stage followed by FT4 determination by RIA. Although FT4 by equilibrium dialysis is less affected by non-thyroidal illness and autoantibodies than Total-T4, it does have limitations, including: it is expensive and labour intensive to perform. FT4 by equilibrium dialysis is not commonly used as an initial screening test for thyroid disease, but rather is best used in those patients whose Total-T4 results appear not be in keeping with their clinical symptoms and where there is a suspicion that the Total-T4 concentration is being affected by autoantibodies or concurrent non-thyroidal illness or drug therapy.

General information about when this test is indicated:

See “Diagnosis and Monitoring of Hypothyroidism” and “Diagnosis and Monitoring of Hyperthyroidism”.

Comparison with other related tests:

See “Diagnosis and Monitoring of Hypothyroidism” and “Diagnosis and Monitoring of Hyperthyroidism”.