Free Tri-iodothyronine (FT3)
Free Tri-iodothyronine (FT3)
Free tri-iodothyronine (FT3) is the active form of thyroid hormone which is critical for body metabolism.
Free tri-iodothyronine (FT3) is the active form of thyroid hormone which is critical for body metabolism.
Free Tri-iodothyronine (FT3)
Normal range
Normal range
3.1-6.8 pmol/L
3.1-6.8 pmol/L
Normal range
3.1-6.8 pmol/L
3.1-6.8 pmol/L
Normal range
Free Tri-iodothyronine (FT3)
Free Tri-iodothyronine (FT3)
Free tri-iodothyronine (FT3) is the active form of thyroid hormone which is critical for body metabolism.
Free Tri-iodothyronine (FT3)
Normal range
3.1-6.8 pmol/L
Normal range
3.1-6.8 pmol/L
3.1-6.8 pmol/L
Normal range
Free Tri-iodothyronine (FT3)
Free Tri-iodothyronine (FT3)
Free tri-iodothyronine (FT3) is the active form of thyroid hormone which is critical for body metabolism.
Free Tri-iodothyronine (FT3)
Normal range
3.1-6.8 pmol/L
Normal range
3.1-6.8 pmol/L
3.1-6.8 pmol/L
Normal range
Free Tri-iodothyronine (FT3)
Dr. Jack Devin
MBBS
Your Body's Most Potent Thyroid Hormone
Your Body's Most Potent Thyroid Hormone
Free Tri-iodothyronine (FT3) is the active, unbound form of T3 - the most potent thyroid hormone in your body. While your thyroid gland produces mostly T4, approximately 80% of T3 is created when T4 is converted in your tissues, particularly in your liver, kidneys, and muscles. FT3 is roughly four times more biologically active than T4, making it the hormone that directly powers your cellular metabolism. This "free" T3 circulates unbound in your bloodstream, ready to enter cells and trigger the metabolic processes that keep you energised, mentally sharp, and physically healthy.
Free Tri-iodothyronine (FT3) is the active, unbound form of T3 - the most potent thyroid hormone in your body. While your thyroid gland produces mostly T4, approximately 80% of T3 is created when T4 is converted in your tissues, particularly in your liver, kidneys, and muscles. FT3 is roughly four times more biologically active than T4, making it the hormone that directly powers your cellular metabolism. This "free" T3 circulates unbound in your bloodstream, ready to enter cells and trigger the metabolic processes that keep you energised, mentally sharp, and physically healthy.
When FT3 Levels Are Too Low
When FT3 Levels Are Too Low
When FT3 Levels Are Too Low
Discover how insufficient active thyroid hormone creates persistent fatigue and metabolic dysfunction despite normal T4 levels.
Read more
When FT3 Levels Are Too High
When FT3 Levels Are Too High
When FT3 Levels Are Too High
Learn why excess T3 creates dangerous metabolic acceleration and the serious cardiovascular risks that follow.
Read more
What Influences Your FT3 Levels?
What Influences Your FT3 Levels?
What Influences Your FT3 Levels?
From conversion problems to medications, understand the complex factors that affect your body's most powerful metabolic hormone.
Read more
Understanding Your Results
Understanding Your Results
Understanding Your Results
Decode your FT3 numbers and discover what they reveal about your thyroid function and cellular metabolism.
Read more
When FT3 Levels Are Too Low
Low FT3 levels can occur even when TSH and FT4 appear normal. Usually, FT3 is aligned with FT4 and low levels may correspond to hypothyroidism. However, if there is a big discrepancy between FT4 and FT3, a specialist hormone doctor should be involved. Borderline high or low levels may not be clinically significant.
Symptoms of low FT3 include persistent fatigue that doesn't respond to rest, brain fog and difficulty concentrating, cold intolerance, especially in hands and feet, depression or low mood, slow metabolism and difficulty losing weight, muscle weakness and joint pain, poor exercise tolerance, and hair loss or thinning. People with low FT3 may describe feeling like they're "running on half power".
Low FT3 can also contribute to elevated cholesterol levels, reduced heart rate variability, compromised immune function, and in some cases, fertility problems.
When FT3 Levels Are Too Low
Low FT3 levels can occur even when TSH and FT4 appear normal. Usually, FT3 is aligned with FT4 and low levels may correspond to hypothyroidism. However, if there is a big discrepancy between FT4 and FT3, a specialist hormone doctor should be involved. Borderline high or low levels may not be clinically significant.
Symptoms of low FT3 include persistent fatigue that doesn't respond to rest, brain fog and difficulty concentrating, cold intolerance, especially in hands and feet, depression or low mood, slow metabolism and difficulty losing weight, muscle weakness and joint pain, poor exercise tolerance, and hair loss or thinning. People with low FT3 may describe feeling like they're "running on half power".
Low FT3 can also contribute to elevated cholesterol levels, reduced heart rate variability, compromised immune function, and in some cases, fertility problems.
When FT3 Levels Are Too Low
Low FT3 levels can occur even when TSH and FT4 appear normal. Usually, FT3 is aligned with FT4 and low levels may correspond to hypothyroidism. However, if there is a big discrepancy between FT4 and FT3, a specialist hormone doctor should be involved. Borderline high or low levels may not be clinically significant.
Symptoms of low FT3 include persistent fatigue that doesn't respond to rest, brain fog and difficulty concentrating, cold intolerance, especially in hands and feet, depression or low mood, slow metabolism and difficulty losing weight, muscle weakness and joint pain, poor exercise tolerance, and hair loss or thinning. People with low FT3 may describe feeling like they're "running on half power".
Low FT3 can also contribute to elevated cholesterol levels, reduced heart rate variability, compromised immune function, and in some cases, fertility problems.
When FT3 Levels Are Too High
Significantly elevated FT3 levels indicate T3 toxicosis, a form of hyperthyroidism. High FT3 overstimulates cellular metabolism, creating symptoms of hyperthyroidism. Borderline high levels may not be clinically significant.
Symptoms typically include rapid, uncontrolled weight loss, severe anxiety, panic attacks, or agitation, racing heart, palpitations, or irregular heartbeat, excessive sweating and heat intolerance, trembling hands and muscle weakness, severe insomnia or sleep disturbances, frequent bowel movements or diarrhoea, and mood swings or irritability.
Long-term elevation can cause irregular heart rhythms, heart failure, osteoporosis from accelerated bone loss, and in severe cases, mental health disorders.
When FT3 Levels Are Too High
Significantly elevated FT3 levels indicate T3 toxicosis, a form of hyperthyroidism. High FT3 overstimulates cellular metabolism, creating symptoms of hyperthyroidism. Borderline high levels may not be clinically significant.
Symptoms typically include rapid, uncontrolled weight loss, severe anxiety, panic attacks, or agitation, racing heart, palpitations, or irregular heartbeat, excessive sweating and heat intolerance, trembling hands and muscle weakness, severe insomnia or sleep disturbances, frequent bowel movements or diarrhoea, and mood swings or irritability.
Long-term elevation can cause irregular heart rhythms, heart failure, osteoporosis from accelerated bone loss, and in severe cases, mental health disorders.
When FT3 Levels Are Too High
Significantly elevated FT3 levels indicate T3 toxicosis, a form of hyperthyroidism. High FT3 overstimulates cellular metabolism, creating symptoms of hyperthyroidism. Borderline high levels may not be clinically significant.
Symptoms typically include rapid, uncontrolled weight loss, severe anxiety, panic attacks, or agitation, racing heart, palpitations, or irregular heartbeat, excessive sweating and heat intolerance, trembling hands and muscle weakness, severe insomnia or sleep disturbances, frequent bowel movements or diarrhoea, and mood swings or irritability.
Long-term elevation can cause irregular heart rhythms, heart failure, osteoporosis from accelerated bone loss, and in severe cases, mental health disorders.
What Influences Your FT3 Levels?
Factors That Can Lower FT3: Several conditions can reduce FT3: chronic illness or severe stress (causing "sick euthyroid syndrome"), certain medications, nutritional deficiencies, and genetic predisposition..
Calorie restriction and extreme dieting can also impact T3 production and overall thyroid function.
Factors That Can Raise FT3: Various conditions can elevate FT3 levels: Graves' disease, toxic thyroid nodules, thyroid medications, thyroiditis, and certain medications that can trigger hyperthyroidism.
What Influences Your FT3 Levels?
Factors That Can Lower FT3: Several conditions can reduce FT3: chronic illness or severe stress (causing "sick euthyroid syndrome"), certain medications, nutritional deficiencies, and genetic predisposition..
Calorie restriction and extreme dieting can also impact T3 production and overall thyroid function.
Factors That Can Raise FT3: Various conditions can elevate FT3 levels: Graves' disease, toxic thyroid nodules, thyroid medications, thyroiditis, and certain medications that can trigger hyperthyroidism.
What Influences Your FT3 Levels?
Factors That Can Lower FT3: Several conditions can reduce FT3: chronic illness or severe stress (causing "sick euthyroid syndrome"), certain medications, nutritional deficiencies, and genetic predisposition..
Calorie restriction and extreme dieting can also impact T3 production and overall thyroid function.
Factors That Can Raise FT3: Various conditions can elevate FT3 levels: Graves' disease, toxic thyroid nodules, thyroid medications, thyroiditis, and certain medications that can trigger hyperthyroidism.
Understanding Your Results
FT3 levels are measured in picomoles per litre (pmol/L):
Low: ≤ 3.1 pmol/L
Normal: 3.11-6.8 pmol/L
High: >6.8 pmol/L
FT3 results are most meaningful when interpreted alongside TSH and FT4 levels. In healthy individuals, FT3 typically mirrors FT4 levels, but this relationship can be disrupted in various conditions. Some people may have normal TSH and FT4 but low FT3, possibly indicating poor T4 to T3 conversion.
FT3 levels can fluctuate more than other thyroid hormones due to their shorter half-life and the complex conversion process. Factors like timing of medication, recent meals, stress levels, and time of day can all influence results.
Understanding Your Results
FT3 levels are measured in picomoles per litre (pmol/L):
Low: ≤ 3.1 pmol/L
Normal: 3.11-6.8 pmol/L
High: >6.8 pmol/L
FT3 results are most meaningful when interpreted alongside TSH and FT4 levels. In healthy individuals, FT3 typically mirrors FT4 levels, but this relationship can be disrupted in various conditions. Some people may have normal TSH and FT4 but low FT3, possibly indicating poor T4 to T3 conversion.
FT3 levels can fluctuate more than other thyroid hormones due to their shorter half-life and the complex conversion process. Factors like timing of medication, recent meals, stress levels, and time of day can all influence results.
Understanding Your Results
FT3 levels are measured in picomoles per litre (pmol/L):
Low: ≤ 3.1 pmol/L
Normal: 3.11-6.8 pmol/L
High: >6.8 pmol/L
FT3 results are most meaningful when interpreted alongside TSH and FT4 levels. In healthy individuals, FT3 typically mirrors FT4 levels, but this relationship can be disrupted in various conditions. Some people may have normal TSH and FT4 but low FT3, possibly indicating poor T4 to T3 conversion.
FT3 levels can fluctuate more than other thyroid hormones due to their shorter half-life and the complex conversion process. Factors like timing of medication, recent meals, stress levels, and time of day can all influence results.
The Takeaway
Free Tri-iodothyronine (FT3) is your body's potent thyroid hormone and the final common pathway for thyroid function. While TSH and FT4 provide important information about your thyroid system, FT3 tells you whether your cells are actually receiving the active hormone they need for optimal metabolism.
Understanding your FT3 levels in the context of your whole thyroid profile is particularly important if you're experiencing unexplained fatigue, weight changes, or other thyroid-related symptoms. Monitoring of FT3 alongside TSH and FT4, gives you and your healthcare provider the complete thyroid picture.
The Takeaway
Free Tri-iodothyronine (FT3) is your body's potent thyroid hormone and the final common pathway for thyroid function. While TSH and FT4 provide important information about your thyroid system, FT3 tells you whether your cells are actually receiving the active hormone they need for optimal metabolism.
Understanding your FT3 levels in the context of your whole thyroid profile is particularly important if you're experiencing unexplained fatigue, weight changes, or other thyroid-related symptoms. Monitoring of FT3 alongside TSH and FT4, gives you and your healthcare provider the complete thyroid picture.
The Takeaway
Free Tri-iodothyronine (FT3) is your body's potent thyroid hormone and the final common pathway for thyroid function. While TSH and FT4 provide important information about your thyroid system, FT3 tells you whether your cells are actually receiving the active hormone they need for optimal metabolism.
Understanding your FT3 levels in the context of your whole thyroid profile is particularly important if you're experiencing unexplained fatigue, weight changes, or other thyroid-related symptoms. Monitoring of FT3 alongside TSH and FT4, gives you and your healthcare provider the complete thyroid picture.
References
British Thyroid Foundation. (2023). Free T3 testing: Clinical significance and interpretation in thyroid disorders. Thyroid function assessment guidelines.
National Institute for Health and Care Excellence. (2023). Thyroid disease: assessment and management. NICE guideline [NG145].
Bianco, A. C., Salvatore, D., Gereben, B., et al. (2002). Biochemistry, cellular and molecular biology, and physiological roles of the iodothyronine selenodeiodinases. Endocrine Reviews, 23(1), 38-89.
Gullo, D., Latina, A., Frasca, F., et al. (2011). Levothyroxine monotherapy cannot guarantee euthyroidism in all athyreotic patients. PLoS One, 6(8), e22552.
References
British Thyroid Foundation. (2023). Free T3 testing: Clinical significance and interpretation in thyroid disorders. Thyroid function assessment guidelines.
National Institute for Health and Care Excellence. (2023). Thyroid disease: assessment and management. NICE guideline [NG145].
Bianco, A. C., Salvatore, D., Gereben, B., et al. (2002). Biochemistry, cellular and molecular biology, and physiological roles of the iodothyronine selenodeiodinases. Endocrine Reviews, 23(1), 38-89.
Gullo, D., Latina, A., Frasca, F., et al. (2011). Levothyroxine monotherapy cannot guarantee euthyroidism in all athyreotic patients. PLoS One, 6(8), e22552.
References
British Thyroid Foundation. (2023). Free T3 testing: Clinical significance and interpretation in thyroid disorders. Thyroid function assessment guidelines.
National Institute for Health and Care Excellence. (2023). Thyroid disease: assessment and management. NICE guideline [NG145].
Bianco, A. C., Salvatore, D., Gereben, B., et al. (2002). Biochemistry, cellular and molecular biology, and physiological roles of the iodothyronine selenodeiodinases. Endocrine Reviews, 23(1), 38-89.
Gullo, D., Latina, A., Frasca, F., et al. (2011). Levothyroxine monotherapy cannot guarantee euthyroidism in all athyreotic patients. PLoS One, 6(8), e22552.
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