Includes T3 (Triiodothyronine), T4 (Thyroxine) and TSH (Thyroid Stimulating Hormone)
Preferred Specimen(s) 1 mL serum
Transport Container Sterile plastic screw-cap vial
Transport Temperature
- Ambient 4 days,
- Refrigerated 14 days
Reject Criteria
- Gross hemolysis
- Gross lipemia
Methodology Chemiluminescence
Clinical Significance
- The degree of suppression of TSH does not always reflect the severity of the hyperthyroidism. Therefore a measurement of free thyroid hormone levels is usually required in patients with a suppressed TSH level. TSH level mild to modest decreases in patients with normal T4 and T3 levels indicates sub-clinical hyperthyroidism.
- Free T4 assays generally are considered to provide the more reliable indication of true thyroid status because only the free hormone is physiologically active. In developing hypothyroidism, T4 (Free T4) is the more sensitive indicator of developing disease and is therefore preferred for confirming hypothyroidism that has already been suggested by an elevated TSH result.
- The T3 is increased in almost all cases of hyperthyroidism and goes up before the T4 is elevated. Thus T3 levels are more sensitive indicator of hyperthyroidism than the total T4. T3 levels are therefore preferred for confirming hyperthyroidism that has already been suggested by a suppressed TSH Levels.
- The Total T3 and T4 hormone concentration is dependent on the concentrations of thyroid binding proteins like Thyroid Binding Globulin (TBG), albumin and Thyroid Binding pre- albumin (transthyretin). Thus, any conditions that affect the levels of thyroid binding proteins will effect these hormone levels.