The fT3 value refers to the concentration of free triiodothyronine.The two thyroid hormones, T3 (triiodothyronine; triiodothyronine) and T4 (thyroxine), are present in protein-bound form and become biologically active when needed by conversion to the free form. In the laboratory, this free form is measured. T3 has a fivefold stronger effect than T4 and 80% of it is produced outside the thyroid gland from T4 (so-called conversion). The biological half-life is about 19 hours. For T4, it is ten times that.
The process
Synonyms
- FT3
- Triiodothyronine
Material needed
- Blood serum
Normal values for fT3
| Adult | 3.4 -7.2 pmol/l |
| Pregnancy |
|
| Children (13-18 years) | 5.2-8.6 pmol/l |
| Children (7-13 years) | 6.2-9.5 pmol/l |
| Children (1-7 years) | 5.2-10.2 pmol/l |
| Infants (1-12 months of age). | 5.1-10.0 pmol/l |
| Neonates (3rd-30th day of life). | 4.3-10.6 pmol/l |
| Newborns (1st and 2nd day of life). | 5.2-14.3 pmol/l |
| Newborn (umbilical cord blood) | 1.6-3.2 pmol/l |
Conversion: ng/l x 1.54 = pmol/l
Interpretation
Several typical constellations of fT4 and TSH are shown below.
- fT4 ↑ or fT3 ↑ and TSH↓
- Hyperthyroidism (overactive thyroid gland).
- Isolated T3 hyperthyroidism (about 10% of cases).
- fT4 ↓ and TSH ↑
- Hypothyroidism (underactive thyroid gland).
- fT4 ↑ or fT3 ↑ and non-suppressed TSH (inadequate TSH secretion).
- Short-term change in disease progression or therapy (dose change of antithyroid drugs, L-thyroxine).
- Pituitary SD hormone resistance
- TSH-producing pituitary tumor (very rare).
- Deficiency of pituitary type II deionidase (very rare).
- fT4 ↑, TSH normal (euthyroid hyperthyroxinemia).
- L-thyroxine substitution therapy
- High-dose beta blocker
- fT3 ↓ (possibly also fT4 ↓) and TSH normal.
- Common in severe general illness (non-tyroid-illness = NTI).
- Euthyroid metabolic state (normal thyroid function) → no substitution required!
- Cause may be drugs that affect the T4 to T3 conversion.
- fT4 ↓ and TSH normal or ↓
- Pituitary secondary hypothyroidism (very rare).
Causes
Hyperthyroidism (hyperthyroidism).
- M. Graves’ disease (about 40%)
- Functional autonomy (30-50%)
- Iodine-induced (contrast media, amiodarone).
- Thyroiditis (initial passive hyperthyroidism possible).
- Iatrogenic or patient-induced (hyperthyroidism factitia) (very rare).
- Hyperthyroidism in differentiated thyroid carcinoma (extremely rare).
- Inadequate TSH secretion (HVL adenoma, paraneoplastic) (extremely rare).
Hypothyroidism (underactive thyroid gland).
- Hashimoto’s thyroiditis
- Frequently iatrogenic (thyrostatic drugs, jodexcess, lithium, condition after SD surgery or radioiodine therapy).
- Congenital hypothyroidism
- Secondary hypothyroidism due to TSH deficiency (rare).
Gravidity (pregnancy)
- TBG ↑ (thyroxine-binding globulin) from the 11th/12th SSW → T4 (total) ↓
- Mostly (relative) iodine deficiency → fT4 (thyroxine) falls continuously down to approx. 0.5 ng/dl
- From the 3rd trimester – T3 ↑ (triiodotyronine) increases to 1.5 times, normalization occurs in the 1st postpartum week
Further notes
- When taking blood in the morning to check an L-thyroxine substitution, the thyroid medication should be taken 24 hours ago, because elevated values can occur shortly after taking the medication.
