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Free triiodothyronine (fT3)

Dr. Jonathan Apasu

Testosteron Experte von Adon Health

The active thyroid hormone for energy and metabolism

Free triiodothyronine (fT3) is the most biologically active thyroid hormone; it determines how fast your cells work, how efficiently energy is converted, and how your body feels overall: energetic or exhausted, focused or slowed down.

For men, fT3 is particularly important because it not only regulates metabolism but is also closely linked to testosterone, libido, fat burning , and mental clarity . Anyone who experiences fatigue, lacks motivation, or is overweight despite "normal" blood test results should always keep an eye on their fT3 levels.

What is fT3?

fT3 stands for free triiodothyronine – the active form of the thyroid hormone T3, which is not bound to proteins and can therefore enter cells directly. It is primarily produced by the conversion of fT4 to fT3 (peripheral conversion), mainly in the liver, kidneys, and intestines.

Only fT3 can increase the cell metabolism rate - it is, so to speak, the "ignition key" for your energy balance.

Why is fT3 so important for men?

A balanced fT3 level promotes:

  • physical energy & vitality
  • clear, focused thoughts
  • regulated fat metabolism
  • healthy libido & sexual function
  • stable mood & motivation
  • optimal effect of testosterone on target cells

Low free T3 (fT3) levels – even with normal TSH or fT4 levels – are associated with fatigue, hormone resistance, and burnout symptoms . In men, it can also significantly reduce the effectiveness of testosterone replacement therapy (TRT).

Signs of an fT3 deficiency

Even within the “normal range”, a low fT3 level can lead to noticeable limitations:

  • Chronic fatigue despite sufficient sleep
  • depressed mood or lack of motivation
  • slow metabolism, weight gain
  • cold hands/feet, sensitivity to cold
  • Brain fog, concentration problems
  • Loss of libido
  • Reduced recovery capacity after exercise
  • reduced effect of testosterone or DHEA

It often occurs in conjunction with elevated cortisol, low ferritin, and an imbalanced intestinal environment.

Causes of low fT3

A low fT3 level is often not due to the thyroid gland itself, but to impaired conversion processes (conversion of fT4 to fT3).

Typical causes:

  • chronic stress / elevated cortisol levels
  • Liver or bowel problems
  • Calorie deficiency, crash diets
  • Selenium or zinc deficiency
  • chronic inflammation
  • High reverse T3 level (RT3 (i.e., in simplified terms, T3 with reversed structural form) blocks T3 receptors)

Men with a “normal thyroid” can suffer from low T3 syndrome – a functional deficiency that is often overlooked in classic laboratory tests.

How is fT3 measured?

fT3 is measured in blood serum. The value becomes meaningful when combined with:

  • fT4
  • TSH
  • Reverse T3 (RT3)
  • Cortisol
  • SHBG & Testosteron
  • Liver and inflammation markers (e.g. CRP, GPT, GGT)

Recommendation: Get tested in the morning on an empty stomach - ideally as part of a comprehensive hormone and metabolism check.

Optimal fT3 levels for men

  • Reference range: approx. 2.0 - 4.4 pg/ml
  • Optimal: In the upper third of the reference range
  • Symptoms often appear even at values ​​below 3.0 pg/ml

Especially for active, performance-oriented men, the lower normal range is often insufficient , and many affected individuals only benefit from values ​​> 3.5 pg/ml.

How can I increase fT3?

Lifestyle & Nutrition

  • Reduce stress : Cortisol lowers T3
  • Eat enough : avoid a chronic calorie deficit
  • A diet rich in protein and minerals (e.g. sufficient tyrosine, selenium, iodine, zinc)
  • Strengthen digestion : A healthy gut flora supports conversion

Micronutrients

  • Selenium (central for fT4→fT3 conversion)
  • Zinc, magnesium, iron, vitamin D
  • Omega-3 fatty acids for inflammation reduction

Medical options

  • fT4 substitution (L-thyroxine) only helps if the conversion is functioning (only with medical supervision)
  • In special cases: Combination therapy with natural T3 (only with medical supervision)

Conclusion

Free triiodothyronine is one of the most important markers for your actual energy level - and often one of the missing building blocks in men who, despite "good blood values", do not feel any performance.

Anyone who feels exhausted, sluggish, or hormonally "blocked" should not overlook their fT3 levels and should take targeted measures to bring them to a functionally optimal level.

Tip: Combine your fT3 analysis with a holistic hormone test - for example, the TRT Check from Adon Health - for a well-founded assessment of your overall metabolic situation.

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