T4 Lab Test

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Latest Edit: Iva Lloyd, ND 2021-08-24 (EDT)

See Also Lab Tests

Thyroxine or T4 is the major hormone secreted by the thyroid gland. The production and secretion of T4 from the thyroid gland is stimulated by TSH or thyroid-stimulating hormone. T4 consists of a tyrosine molecule with 4 iodine atoms attached to it. The active thyroid hormone T3 is formed from the cleavage of an iodine molecule from T4. The majority of T4 is transported through the blood by being bound to thyroxine-binding globulin (TBG), pre-albumin, and albumin.[1],[2]


  • T4 conversion into T3 is dependent upon the presence of iodine, selenium, and tyrosine amongst other nutrients
  • Thyroxine has a wide range of metabolic activities in the body:
    • sets the basal metabolic rate
    • Normal growth and development
    • Metabolism of fats and proteins
    • Regulation of stress hormones and neurotransmitters
    • Immune resistance

Patient Preparation

  • No fasting is required
  • Factors which can cause increased levels
  • Pregnancy- especially 2nd and 3rd trimester due to increased estrogen levels, thyroid treatment within one month of testing
  • Drugs: estrogen, methadone, tamoxifen, and oral contraceptives
  • Factos which can cause decreased levels: None noted
  • Drugs: Steroids, androgens, danazol, phenytoin, and propanolol

Clinical Implications

Ranges: The following are the reference ranges for this lab. However, lab ranges can vary by laboratory and country. [2]

Standard U.S. Units Standard International Units
Conventional Laboratory Range 4.8-13.2 mcg/dL 61.8-169.9 mmol/L
Optimal Range 6.0-12.0 mcg/dL 77.2- 154.4 mmol/L
Alarm Ranges <5.0 or > 13.0 mcg/dL <65 or > 167 nmol/L

High levels indicate:

Low levels indicate:

Associated Tests


  1. Pagana Kathleen D, Pagana Timothy J (1998) Mosby's Manual of Diagnostic and Laboratory Tests, Mosby, Inc
  2. 2.0 2.1 Weatherby Dicken, Ferguson Scott (2002) Blood Chemistry and CBC Analysis: Clinical Laboratory Testing from a Functional Perspective, Bear Mountain