The fT4 value refers to the concentration of free thyroxine. The two thyroid hormones, T3 (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
- FT4
- Thyroxine
Material needed
- Blood serum
Normal values for fT4
| Adult | 0.73-1.95 ng/dl (9.4-25 pmol/l) |
| Pregnancy |
|
| Children (13-18 years) | 0.9-1.8 ng/dl |
| Children (7-13 years) | 0.9-1.7 ng/dl |
| Children (1-7 years) | 0.9-1.7 ng/dl |
| Infants (1-12 months of age). | 1.1-1.8 ng/dl |
| Neonates (3rd-30th day of life). | 1.5-3.0 ng/dl |
| Neonates (1st and 2nd day of life). | 1.6-3.8 ng/dl |
| Newborn (umbilical cord blood) | 1.0-1.8 ng/dl |
Conversion factor
- Ng/dl x 12.87 = pmol/l
In pregnant women, T4 levels generally drop to levels of circa 0.5 ng/dl after the 12th week of gestation. This is caused by a relative iodine deficiency.
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).
- Elevation of thyroxine-binding protein (TBG) or transthyretin (TTR, thyroxine-binding prealbumin, TBPA).
- L-thyroxine substitution therapy
- High-dose propranolol (beta blocker)
- fT3 ↓ (possibly also fT4 ↓) and TSH normal.
- Common in severe general illness (non-tyroid-illness = NTI).
- Euthyroid metabolic state (thyroid function is normal ) → 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 (inflammation of the thyroid gland; 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 (congenital hypothyroidism).
- Secondary hypothyroidism due to TSH deficiency (rare).
