T4 (THYROXINE), TOTAL
1 SST/1.0 mL serum. After 30 minutes of clotting, promptly centrifuge and refrigerate.
Refrigerated: 1 week; Frozen: 1 month
Thyroxine binding globulin (TBG) concentrations remain reasonably constant in healthy individuals. However, pregnancy, excess estrogens, androgens, anabolic steroids, and glucocorticoids are known to alter TBG levels and may cause false thyroid values for thyroid function tests. Altered T4 levels in these situations may not accurately reflect thyroid status. Primary malfunction of the thyroid gland may result in excessive (hyper) or below normal (hypo) release of T4 or T3. In addition, as thyroid function is directly affected by TSH, malfunction of the pituitary or they hypothalamus influences the thyroid gland activity. Disease in any portion of the thyroid-pituitary-hypothalamus system may influence the levels of T4 and T3 in the blood.