Wallach's Interpretation of Diagnostic Tests: Pathways to Arriving at a Clinical Diagnosis (1248 page)

   Both free and bound forms of T
₄
and T
₃
are present in the blood. More than 99% of the T
₄
and T
₃
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. FT
₄
is usually 0.02–0.04% of total T
₄
(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

   FT
₄
gives corrected values in patients in whom the total T
₄
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.