Both free and bound forms of T4 and T3 are present in the blood. More than 99% of the T4 and T3 circulates in the blood bound to carrier proteins, leaving <1% unbound. It is this level of unbound or free hormone that correlates with the functional thyroid state in most individuals. FT4 is usually 0.02 " “0.04% of total T4 (see Table 16.76).
Normal range (adults): 0.58 " “1.64 ng/dL.
Pregnant women:
First trimester: 0.73 " “1.13 ng/dL
Second trimester: 0.54 " “1.18 ng/dL
Third trimester: 0.56 " “1.09 ng/dL
Use
FT4 gives corrected values in patients in whom the total T4 is altered on account of changes in serum proteins or in binding sites (e.g., pregnancy, drugs [such as androgens, estrogens, birth control pills, phenytoin], altered levels of serum proteins [e.g., nephrosis]).
Monitoring restoration to normal range is the only laboratory criterion to estimate appropriate replacement dose of levothyroxine because 6 " “8 weeks are required before TSH reflects these changes.
Not generally helpful unless pituitary/hypothalamic disease is suspected.
Interpretation
Increased In
Hyperthyroidism.
Hypothyroidism treated with thyroxine.
Euthyroid sick syndrome.
Occasional patients with hydatidiform mole or choriocarcinoma with marked hCG elevations may show increased FT4, suppressed TSH, and blunted TSH response to TRH stimulation; returns to normal with effective treatment of trophoblastic disease; severe dehydration (may be >6.0 ng/dL).
Decreased In
Hypothyroidism
Hypothyroidism treated with triiodothyronine
Euthyroid sick syndrome
Limitations
FT4 assays based on direct equilibrium dialysis are considered reference methods.
FT4 assays are prone to inaccurate readings in pregnant women. The studies have shown that FT4 index measurement is more reliable than free T4 immunoassays in pregnant women.
Anticonvulsant drug therapy (particularly phenytoin) may result in decreased FT4 levels due to an increased hepatic metabolism and secondarily to displacement of hormone from binding sites.