Wallach's Interpretation of Diagnostic Tests: Pathways to Arriving at a Clinical Diagnosis (1250 page)

T
₃
, triiodothyronine; T
₄
, thyroxine; NTI, nonthyroid illness.
*Low TSH with normal T
₄
.
^†
High TSH with normal T
₄
.
^‡
In 95% of cases of thyrotoxicosis. Serum T
₃
is needed for diagnosis of T3 thyrotoxicosis in the other 5% of cases of thyrotoxicosis.

   Use

   Reflects secretory activity; useful in diagnosis of hyper- and hypothyroidism, especially when overt or due to pituitary or hypothalamic disease

   
Normal range:
6.09–12.23 μg/dL

   Interpretation

   Not affected by

   Mercurial diuretics

   Nonthyroidal iodine

Increased In

   Hyperthyroidism.

   Pregnancy.

   Drugs (e.g., estrogens, birth control pills,
D
-thyroxine, thyroid extract, TSH, amiodarone, heroin, methadone, amphetamines, some radiopaque substances for x-ray studies [ipodate, iopanoic acid]).

   Euthyroid sick syndrome.

   Increase in TBG or abnormal thyroxine-binding prealbumin.