T3 Lab Test

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.,

Discussion

• 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.

	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:

• Hypothyroidism
• Iodine deficiency
• Other conditions: protein malnutrition, renal disease, liver disease

Low levels indicate:

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

Associated Tests

• Serum T-3 uptake, T4, free thyroxine index (FTI), thyroid stimulating hormone (TSH), thyroid binding globulin (TBG), serum triglycerides, serum cholesterol, serum calcium, serum phosphorous, RBC magnesium, serum albumin, HCT, HGB