Table 211

Laboratory Findings Used for Diagnosis of Thyroid Disorders

Disorder

Mechanism

RT3U* FT4 lndex+ TS H

Graves disease

Secondary hyperthyroidism

Hashimoto thyroiditis

Primary hypothyroidism

Secondary hypothyroidism

Estrogen effect

Production of TSH High receptor-stimulating autoantibodies

TSH adenoma High

Production of thyroid Low peroxidase autoantibodies

Production of TSH Low receptor-blocking autoantibodies

Low TSH secretion by Low adenohypophysis or low TRF secretion by the hypothalamus

Oral contraceptives, High

DES therapy for prostate cancer

High High

Undetectable

High High High

Low Low High

Very high

Normal Normal

DES = diethylstilbestrol; FT4 = free thyroxine; RT3U = resin T3 uptake test; T3 = triiodothyronine; T4 = thyroxine; TRF = thyrotropin-releasing factor; TSH = thyroid-stimulating hormone.

♦The resin T3 uptake (RT3U) test is not a measure of serum T3 levels; rather, it evaluates thyroid-binding globulin (TBG) levels via a competition assay between a resin and TBG for radioactive T3. If TBG levels are low, then more radioactive T3 will bind to the resin.

fThis index is a measure of the true free4 in the serum. It is calculated by multiplying the serum T4 level by the RT3U.

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