Thyroxine, Total (T4)

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Subject: Thyroxine, Total (T4)

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Definition

  • T4 is major secretion of the thyroid. Bound to TBG, prealbumin, and albumin in blood. In tissues, it is deiodinated to T3, 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.)

Figure 16–5
Algorithm for thyroid function testing.

(D, decreased; I, increased; N, normal.)

 
TABLE 16–79
Free Thyroxine (FT4) and Thyroid-Stimulating Hormone (TSH) Levels in Various Conditions

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.

  • Familial dysalbuminemic hyperthyroxinemia—albumin binds T4 but not T3 more avidly than normal, causing changes similar to thyrotoxicosis (total T4 approximately 20 μg/dL, normal thyroid hormone–binding ratio, increased free T4 index), but the patient is not clinically thyrotoxic.

  • Serum T4 >20 μg/dL usually indicates true hyperthyroidism rather than increased TBG.

  • May be found in euthyroid patients with increased serum TBG.

  • Much higher in first 2 months of life than in normal adults.

Decreased In

  • Hypothyroidism

  • Hypoproteinemia (e.g., nephrosis, cirrhosis)

  • Certain drugs (phenytoin, triiodothyronine, testosterone, ACTH, corticosteroids)

  • Euthyroid sick syndrome

  • Decrease in TBG

Normal In

  • Hyperthyroid patients with

    • T3 thyrotoxicosis

    • Factitious hyperthyroidism owing to T3 (Cytomel)

    • Decreased binding capacity due to hypoproteinemia or ingestion of certain drugs (e.g., phenytoin, salicylates)

Limitations

  • Various drugs can interfere with the test result.

 
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