Total T4 (1015)

Total T4 (thyroxine) is a hormone produced by the thyroid gland and is converted in the body’s tissue to the major active thyroid hormone T3.

Methodology

Chemiluminescence immunoassay

• Low Total T4 levels are consistent with an underactive thyroid gland or inadequate thyroid hormone replacement.1-3
• High Total T4 levels are consistent with either an overactive thyroid gland or excess thyroid hormone replacement.1-3
• High doses of biotin (> 5 mg/day) may interfere with assay results. Patient assumed to be refraining from biotin supplementation for at least 3 days prior to blood draw.

References:

1. Goodman N, Cobin RH. Reproductive disorders. In: Camacho PM, Gharib H, Sizemore GW, eds. Evidence-Based Endocrinology. 3rd ed. Philadelphia, PA: Lippincott Williams & Wilkins. 2012:190-228.
2. Castro MR, Gharib H. Thyroid disorders. In: Camacho PM, Gharib H, Sizemore GW, eds. Evidence-Based Endocrinology. 3rd ed. Philadelphia, PA: Lippincott Williams & Wilkins. 2012:43-80.
3. Garber JR, Cobin RH, Gharib H, et al. Clinical practice guidelines for hypothyroidism in adults: cosponsored by the American Association of Clinical Endocrinologists and the American Thyroid Association. Endocr Pract. 2012;18(6):988-1028.