Reverse T3 (rT3) is an inactive form of the thyroid hormone, triiodothyronine (T3). It is produced when the body converts thyroxine (T4) into rT3 instead of the active T3. rT3 competes with active T3 at cellular receptors but does not activate them, essentially blocking the effect of T3.
Although rT3 might seem like a minor character, it has a vital role in your body's complex hormone balance. It works as a natural control system, slowing your metabolism when your body experiences stress, and saving energy for when most necessary. This makes rT3 a key part of your body's stress response, helping to keep a delicate balance in how your body manages energy.
The clinical uses of the rT3 test are limited compared to other thyroid function tests, but it can still provide valuable information.
Abnormal Reverse T3 Levels
Low Reverse T3 (Low rT3) - Causes
Low rT3 levels may signal a dysfunction in your body’s natural metabolic control. Potential causes include:
- Hyperthyroidism: An overactive thyroid can result in lower levels of rT3 as more T4 is converted to active T3.
- Excess thyroid hormone medication: In cases where a patient is overmedicated with thyroid hormone (e.g., T4 or T3), rT3 levels may drop.
- Other medications: beta-blockers or steroids
- Liver or kidney dysfunction: Either may impair the conversion of T4 into T3.
- Nutrient deficiencies (such as iodine or selenium)
- Stress or adrenal fatigue
Low Reverse T3 (Low rT3) - Symptoms
- Unexplained weight loss
- Increased heart rate
- Anxiety or irritability
- Difficulty sleeping
- Feeling unusually warm
Elevated Reverse T3 (High rT3) - Causes
High rT3 levels may indicate your body is trying to conserve energy due to stress or problems converting T4 to T3. Potential causes of elevated rT3 include:
- Stress: Physical or emotional stress, such as from illness, injury, or trauma, can cause elevated rT3.
- Chronic illness: Conditions like heart, kidney, and liver disease are linked to high rT3.
- Low-calorie or starvation diets: Extreme dieting or malnutrition can increase rT3.
- Hypothyroidism: In some cases of low thyroid hormone production, the body may convert more T4 into rT3.
- Adrenal fatigue: Imbalances in cortisol levels can lead to elevated rT3.
- Liver dysfunction: The liver plays a role in converting T4 to T3, and if impaired, more T4 may be converted to rT3.
- Certain medications: Corticosteroids, beta-blockers, and other medications can impact rT3 levels.
Elevated Reverse T3 (High rT3) - Symptoms
- Fatigue
- Weight gain or difficulty losing weight
- Brain fog, difficulty concentrating, or memory issues
- Depression
- Cold intolerance
- Hair loss
- Constipation
- Slow heart rate (bradycardia)
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