T4, Total

Alternative Names
  • Syn-Thyroid
  • T4
  • Serum
  • Tetraiodothyronine
  • Thyroid Hormone
  • Thyroxine
  • Total
  • T4 Total Only
  • Thyroxine
  • Thyroxine (T4)
  • Thyroxine Binding Index-TBI
  • Testosterone-Estradiol Binding Globulin
  • Testosterone-binding Globulin

Test Code: 4017

CPT: 84436
Tests Included Total T4
Use T4 and T3 regulate normal growth and development. They maintain body temperature, stimulate calorigenesis and affect all aspects of carbohydrate metabolism as well as certain areas of lipid and vitamin metabolism. Fetal and neonatal development also require thyroid hormones.1,2
Clinical Utility Most of the T4 and T3 circulates in the blood bound to protein, while a small percentage is free (not bound). Blood tests can measure total T4 (unbound plus bound), free T4, total T3 (bound plus unbound), or free T3. Since most T3 is bound to protein, the total T3 can be affected by protein levels and protein binding ability, but the free T3 is not.
Low TSH, Normal T4 & T3 - Probably Mild (subclinical) hyperthyroidism
Low TSH, High/Normal T4 and T3, Hyperthyroidism
Low TSH & Low/Normal T4 & T3, Non-thyroidal illness; rare pituitary (secondary hypothyroidism
Normal TSH & High T4 & T3, Thryroid Hormone resistance syndrome (a mutation in the thyroid hormone receptor decreases thyroid homrone High TSH, Normal T4 & T3, Mild (subclinical) hypothyroidism
High TSH, Low T4 & Low/Normal T3, Hypothyroidism
Intended Patient Population 18+ and Older Adult Males & Females
Patient Preparation None Specified
Sample Serum, Plasma
Tube Red, Green, Tiger
Volume 4mL Whole Blood (1mL Serum/Plasma)
Min Sample Volume 0.5 mLs
Reference Ranges M & F ≥ 18 yrs old; 5.18-10.86 ug/dL
Analytical Measurement Range 0.50-30.00 ug/dL
Units ug/dL
Test Methodology Chemiluminescent Immunoassay
Test Turnaround Time 2 Days
Limitations Changes in binding proteins can occur which affect the level of total T4 but leave the level of unbound hormone unchanged.2

In pregnancy, Total T4 results may be incorrect, i.e., falsely-low. This assay should not be used as the only marker for thyroid disease evaluation during pregnancy. To ensure maximum diagnostic accuracy, thyroid status in pregnant women should be determined using thyroid function tests such as TSH, Free T4, Free Thyroxine Index (FTI) and clinical evaluation by the physician.3
Shipping Requirements Refrigerated
Specimen Stability 7 Days RF
Reject Criteria Gross Hemolysis
Laboratory Developed Test (LDT) Yes
CMS Guidance NCD 190.22
References
  1. Gornall, AG, Luxton, AW, Bhavnani, BR. Endocrine disorders in applied biochemistry of clinical disorders, 305-318. Edited by Gornall, AG Philadelphia, PA: J B Lippincott Co, 1986.
  2. White, GH, Recent advances in routine thyroid function testing, CRC - Critical Reviews in Clinical Laboratory Sciences, 24: 315-362: 1987.
  3. ACOG Practice Bulletin. Clinical management guidelines for obstetrician-gynecologists. Number 37, August 2002.(Replaces Practice Bulletin Number 32, November 2001). Thyroid disease in pregnancy. Obstet Gynecol. 2002 Aug; 100(2): 387-96.

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