Wallach's Interpretation of Diagnostic Tests: Pathways to Arriving at a Clinical Diagnosis (1261 page)

   Not recommended for diagnosis of hypothyroidism; decreased values have minimal clinical significance.

   More than 99% of the total concentration of T
₃
and T
₄
is bound by serum proteins, which is not available to elicit biologic activity. It is only the free fraction (<1%) that is readily available to bind its receptor and stimulate a response from the target organ or tissues.

   Values below the lower limit of the expected value range can be caused by a number of conditions, including nonthyroidal illness, acute and chronic stress, and hypothyroidism.

TRIIODOTHYRONINE (T
₃
) RESIN UPTAKE (RUR)

   Definition

   Measures unoccupied binding sites on TBG. Not a measure of T
₃
concentration, which is assayed by other methods for diagnosis of T
₃
thyrotoxicosis. Now replaced by free T
₄
(see Table 16.77).

   
Normal range:
median value of 40.0% (0.40) with a 95% nonparametric range of 32.0–48.4%.

   Use

   Only with simultaneous measurement of serum T
₄
to calculate T
₇
in order to exclude the possibility that an increased total T
₄
is due to an increase in TBG.

   RUR is inversely proportional to unsaturated hormone-binding sites.

   Total T
₄
× RUR is proportional to free T
₄
and inversely proportional to TSH.

   Interpretation

   Decreases when binding protein increases (pregnancy)

   Increases when binding protein decreases (hyperthyroidism)