Thyroxine (T4)

Test ID:

704318

CPT code:

84436

Synonyms:

T4, Total
Tetraiodothyronine

Clinical Use:

Thyroid function test. Decreased in hypothyroidism and in the third stage of (painful) subacute thyroiditis; increased with hyperthyroidism, with subacute thyroiditis in its first stage and with thyrotoxicosis due to Hashimoto disease.3 Used to diagnose T4 toxicosis.

Test Information:

The combination of the serum T4 and T3 uptake (THBR) as an indirect assessment of TBG, helps to determine whether an abnormal T4 value is due to alterations in serum thyroxine-binding globulin or to changes of thyroid hormone levels. Deviations of both tests in the same direction usually indicate that an abnormal T4 is due to abnormalities in thyroid hormone. Deviations of the two tests in opposite directions provide evidence that an abnormal T4 may relate to alterations in TBG.

Causes of increased TBG binding include neonatal state, molar and conventional pregnancy, estrogens, oral contraceptives, heroin, methadone, 5-fluorouracil, clofibrate, infectious hepatitis, chronic active hepatitis, and primary biliary cirrhosis, acute intermittent porphyria, lymphoma, and hereditary TBG increase.

Amphetamines, iopanoic acid, ipodate, and amiodarone increase thyroxine. High dose propranolol may elevate T4 and FTI levels.

Causes of decreased TBG binding include abnormal protein states especially nephrotic syndrome, androgens, anabolic steroids, prednisone, acromegaly, liver or other systemic illness, severe stress or hereditary TBG deficiency. Salicylates and diphenylhydantoin may lower T4 significantly. Amiodarone may cause increased thyroxine levels and can cause hypothyroidism or hyperthyroidism.

Lithium carbonate may cause goiter with or without hypothyroidism.

Carbamazepine (Tegretol®) is reported to cause decreased values in thyroid function tests.

Specimen Type:

Serum

Requested Volume:

1 mL (adult), 0.8 mL (pediatric

Minimum Volume:

0.5 mL (adult), 0.3 mL (pediatric) (Note: This volume does not allow for repeat testing.)

Container Type:

Red-top tube or gel-barrier tube

Patient Preparation:

It is recommended to ask all patients who may be indicated for this test about biotin supplementation. Patients should be cautioned to stop biotin consumption at least 72 hours prior to the collection of a sample.