What the value means
The thyroid mainly produces T4, which tissues convert into active T3. If TSH and fT4 are normal, both the control and production of hormones usually work, and a low fT3 reflects how the body converts T4. During illness, fasting or heavy strain the body deliberately slows this conversion to save energy – this is called low T3 syndrome or non-thyroidal illness. Reference ranges for fT3 differ between laboratories, and fT3 is measured less precisely than fT4, so a slightly low value should be interpreted cautiously and in context.
Possible causes
- Acute or recent illness – an infection, surgery or inflammation temporarily reduces conversion of T4 to T3.
- Calorie deficit and fasting – prolonged energy restriction, strict diets or very low carbohydrate intake often lower fT3.
- Low energy availability in sport – lots of training and too little food, typically in endurance athletes.
- Chronic stress and little sleep – higher cortisol affects hormone conversion.
- Some medicines – for example corticosteroids or beta blockers can lower fT3.
- Nutrient shortfalls – conversion requires, among others, selenium, iron and zinc.
What you can do
- Enough energy – if you are dieting or fasting, check whether you have been eating too little for too long.
- Balance training – include rest days and eat in line with your training volume.
- Sleep and stress – 7–9 hours of sleep and regular moments of relaxation during the day.
- Varied diet – fish, eggs, nuts, legumes; supplements only for a confirmed deficiency.
- Repeat the test – after 6–8 weeks, when you are well, in the morning and at the same laboratory, together with TSH and fT4.
When to see a doctor
See a doctor if you have marked fatigue, feel cold, lose hair, have irregular or missed periods, unintended weight loss or gain, or if low fT3 persists despite good health and adequate food. Treatment with T3 hormone is not routinely recommended in this situation and belongs solely in the hands of an endocrinologist.