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What is free T3?

T3, or triiodothyronine, is the main active thyroid hormone in the human body. Once released into the bloodstream, TSH is responsible for stimulating the thyroid gland to produce hormones T4 (thyroxine) and T3 (triiodothyronine). T4 is inactive and is converted to the active T3 form, accounting for 80% of total T3 synthesis. While bound T3 (attached to proteins) is unable to enter and act on the body’s tissues, free T3 is able to enter tissues and act more readily.

Why is free T3 important?

Tracking your free T3 levels allows you to understand the health of your thyroid gland, which is an integral part of your endocrine system. Thyroid hormones are vital to metabolism, heart and digestive functions, muscle control, brain development, and bone maintenance, among other roles. Because of its significance, free T3 is included in SiPhox Health‘s Thyroid+ panel.

High free T3 may point to hyperthyroidism (overactive thyroid gland), which can be caused by autoimmune diseases or thyroid gland inflammation (thyroiditis). Low free T3 may point to hypothyroidism (underactive thyroid gland). Symptoms of hyperthyroidism may include weight loss, rapid or irregular heartbeat, increased hunger, and anxiousness among other issues. Symptoms of hypothyroidism on the other hand may include lethargy, weight gain, sensitivity to cold, thinning hair, constipation, and other physical and mental issues. If you are concerned with your levels, it is important to consult with your physician.

How can I better understand my free T3 levels?

According to the American Board of Internal Medicine:

  • Low: <1.9 pg/mL
  • Optimal: 3.2-3.9 pg/mL
  • Normal: 1.9 – 4.4 pg/mL
  • High: >4.4 pg/mL

Free T3 test results are analyzed in conjunction with other thyroid tests to determine whether the thyroid gland is functioning properly.

Like many hormones, T3 follows a circadian rhythm related to the cycles of TSH and T4. Free T3 is generally at its highest concentration at around 4am and at its lowest concentration at around 4pm. As such, testing in the morning compared to in the afternoon may help detect hypothyroidism in individuals who would otherwise appear normal due to rhythmic fluctuation.

How can I maintain optimal free T3 levels?

If you need to maintain or adjust your free T3 levels, consider the following strategies:

Dietary Changes: Consuming less iodine and eating more cruciferous veggies can help in reducing elevated free T3 levels. If your free T3 levels are low, ensure adequate calorie and carb intake.

Lifestyle Changes: If your T3 levels are low, try managing your inflammation, reducing stress, and improving sleep quality and duration.

Where can I learn more?

DISCLAIMER: IF YOU ARE CONCERNED WITH ANY OF YOUR RESULTS, PLEASE CONSULT WITH YOUR PHYSICIAN.