Wallach's Interpretation of Diagnostic Tests: Pathways to Arriving at a Clinical Diagnosis (1249 page)

   Euthyroid sick syndrome.

   Occasional patients with hydatidiform mole or choriocarcinoma with marked hCG elevations may show increased FT
₄
, 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

   FT
₄
assays based on direct equilibrium dialysis are considered reference methods.

   FT
₄
assays are prone to inaccurate readings in pregnant women. The studies have shown that FT
₄
index measurement is more reliable than free T
₄
immunoassays in pregnant women.

   Anticonvulsant drug therapy (particularly phenytoin) may result in decreased FT
₄
levels due to an increased hepatic metabolism and secondarily to displacement of hormone from binding sites.

THYROXINE, TOTAL (T
₄
)

   Definition

   T
₄
is major secretion of the thyroid. Bound to TBG, prealbumin, and albumin in blood. In tissues, it is deiodinated to T
₃
, which causes hormonal action and is responsible for hormonal action. See Tables 16.76 and 16.79 and Figure 16.5.

Figure 16–5
Algorithm for thyroid function testing. (D, decreased; I, increased; N, normal.)

TABLE 16–79. Free Thyroxine (FT
₄
) and Thyroid-Stimulating Hormone (TSH) Levels in Various Conditions