Zoetis – Thyroxine T4/Cholesterol Profile

1. Intended Use

The VetScan T4/Cholesterol reagent rotor used with the VetScan Whole Blood Analyzer utilizes dry and liquid reagents to provide in vitro quantitative determinations of thyroxine (T4) and cholesterol in heparinized whole blood, heparinized plasma, or serum.

2. Summary and Explanation of Tests

Thyroxine (T4)

Thyroxine is a hormone synthesized in and secreted by the thyroid gland. The primary secretory form of the thyroid hormone is tetraiodothyronine (T4), although some triiodothyronine (T3) is also secreted into the blood. The ratio of T4 to T3 is 25:1 in canine plasma. Once in the blood, T4 and T3 are bound by transport proteins. The primary binding protein is thyroxine-binding globulin (TBG) in the dog and albumin in the cat. Upon delivery to the target cell, T4 is deionated to T3 at the cell surface. T3 is the biologically active form of the thyroid hormone and more readily enters the target cell.

Thyroid hormone has many effects on the body, including clinical, physiological, calorigenic, metabolic (carbohydrate, protein and lipid), developmental, reproductive and hematologic. T4 determinations aid in the diagnosis of hypothyroidism and hyperthyroidism and in monitoring sodium levothyroxine and methimazole therapies.

Clinical signs of abnormal T4 levels are often vague. The most common observable signs of canine hypothyroidism are skin and coat changes, such as alpecia or a dry, dull coat. Other signs in dogs include lethargy, exercise intolerance, weakness, muscle atrophy, corneal lipid deposits and diarrhea. Clinical signs of feline hypothyroidism include lethargy and obesity (especially in iatrogenic hypothyroidism), alopecia, epilation of hair and bradycardia.

The most prevalent clinical signs of feline hyperthyroidism are weight loss and polyphagia. Other common signs are restlessness, tachycardia, polyuria-polydipsia, alopecia and diarrhea.

Cholesterol

Cholesterol is a major precursor of cholesterol ester, bile acids and steroid hormones and is a component of plasma membranes. The rate of cholesterol biosynthesis in the liver is indirectly proportional to dietary intake. Levels of cholesterol in the body are indirectly controlled by thyroid hormone, which stimulates bile acid production. Since bile acids are synthesized from cholesterol, cholesterol concentrations vary inversely with thyroid hormone activity.

Cholesterol levels may be used to aid in detection of hyperlipidemia or as a screening test for hypothyroidism and hyperadrenocorticism. Cholesterol results are most useful when analyzed in conjunction with other clinical chemistry tests.

As with any diagnostic test procedure, all other test procedures including the clinical status of the patient should be considered prior to final diagnosis.

Additional Information