T3 Lab Test

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

See Also Lab Tests

T-3 is produced from the conversion of thyroxine (T-4) in the peripheral tissue where approximately one-third of T4 is converted to T-3. T-3 is the most active thyroid hormone. Its structure consists of three iodine atoms attached to the tyrosine molecule, where T4 has four iodine atoms. T-3 is more metabolically active than T4 and its systemic effects are shorter. T-3 will bind to protein less efficiently and for a shorter duration than T4.[1],[2]


  • Measuring T-3 can be useful in diagnosing hypothyroidism and hyperthyroidism (thyrotoxicosis)..
  • T-3 exists in two forms in the serum: the majority of T-3 is bound to protein and less than 1% of the total T-3 is unbound or free T-3.

Patient Preparation

  • Factors which can cause increased levels
  • Following desiccated thyroid medications (several hours)
  • Factors which can cause decreased levels
  • Severe illness

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 80-230 ng/dL 1.23-3.53 nmol/L
Optimal Range 100-230 ng/dL 1.54- 3.53 nmol/L
Alarm Ranges <70 or >230 ng/dL <1.07 or >3.53 nmol/L

High levels indicate:

Low levels indicate:

  • Primary hypothyroidism
  • Selenium deficiency
  • Other conditions: pregnancy, severe liver disease, prescribed drug or radiation therapy for hyperthyroidism

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