para>(D, decreased; I, increased; N, normal.)View OriginalFigure 16 " “5Algorithm for thyroid function testing.
(D, decreased; I, increased; N, normal.)
View Original
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TABLE 16 " “79Free Thyroxine (FT4) and Thyroid-Stimulating Hormone (TSH) Levels in Various ConditionsView LargeTABLE 16 " “79Free Thyroxine (FT4) and Thyroid-Stimulating Hormone (TSH) Levels in Various Conditions Sensitive TSH Normal Low High Normal Euthyroid Subclinical/early hyperthyroidism* Subclinical/early hypothyroidism " NTI NTI Drug effects (e.g., l-dopa, glucocorticoids) Drug effects (e.g., iodine, lithium, antithyroid drugs, amiodarone, interferon alfa) Replacement therapy or excess T4 therapy for hypothyroidism Insufficient T4 therapy for hypothyroidism Rule out thyrotoxicosis First 4 " “6 wk of therapy for hypothyroidism T4 Low Secondary hypothyroidism Secondary hypothyroidism NTI Primary hypothyroidism Drug effects (e.g., T3, phenytoin, androgens, salicylates, carbamazepine, rifampin) Drug effects (e.g., dopamine, T3, corticosteroids) Drug effects (e.g., iodine, lithium, antithyroid drugs, amiodarone) T3 hyperthyroidism (e.g., Graves disease, toxic goiter, thyroiditis, factitious/iatrogenic, hyperthyroidism, struma ovarii, thyroid carcinoma) Insufficient T4 therapy for hypothyroidism High NTI (e.g., psychiatric and acute illness) NTI (e.g., psychiatric and acute illness) TSH-secreting tumor Abnormal binding (excess TBG, familial dysalbuminemic hyperthyroxinemia, some monoclonal proteins) Primary hyperthyroidism " Thyroid hormone resistance Thyroid hormone resistance Drug effects (e.g., estrogen, iodine drugs or contrast media, thyroxine [factitious])
T3, triiodothyronine; T4, thyroxine; NTI, nonthyroid illness.
*Low TSH with normal T4. " High TSH with normal T4. " ¡In 95% of cases of thyrotoxicosis. Serum T3 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 Ž ¼g/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 Ž ¼g/dL, normal thyroid hormone " “binding ratio, increased free T4 index), but the patient is not clinically thyrotoxic.
- Serum T4 >20 Ž ¼g/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.