Free T3 (Triiodothyronine): the main metabolically active thyroid hormone.
Produced by: Thyroid Gland
Purpose: Increases cardiac output, heart rate, ventilation rate, basal metabolic rate, potentiates the effects of catecholamines (for example: neurotransmitters, epinephrine, dopamine), potentiates brain development, thickens the endometrium in females, increases the catabolism of proteins and carbohydrates.
Lab Reference Range: 2.3 – 4.2 pg/mL
When to order:
- To initially screen for hyper or hypothyroidism
- To monitor thyroid medication usage.
Half-life: 6 hours.
When each local area of your body determines that it needs more energy, it goes to your body’s reservoir of T4 and converts it to Free T3, the active thyroid hormone. T3 attaches to receptors inside of your cells to increase metabolism.
If your Free T3 lab test results come back at 2.3 pg/mL, you are technically in the normal range. But isn’t that analogous to achieving a D- on an exam and considering it a passing grade? Some individuals will have symptoms of hypothyroidism (low) when their Free T3 levels are in the low normal range. Consult your health care practitioner.
Lab test results for Free T3 will usually come back elevated if you take your T3 thyroid medication the morning before your lab test.
Free T3 is elevated in Grave’s Disease and initially with Hashimoto’s Thyroiditis.
Thyroxine-binding globulin (TBG) is a protein made in the liver that binds and carries T4 and T3 in the bloodstream.
The thyroid gland produces T4 and T3 in a ratio of 14:1 to 20:1 (so, a lot more T4).
T4 is the reservoir for T3 production.
The prescriptions Cytomel and Liothyronine are pure T3.
Free T4 is converted to T3 in five main places: Liver, Gut, Muscle, Brain, Thyroid gland.