Thyroxine (T4)

CPT: 84436
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  • T4, Total
  • Tetraiodothyronine

Special Instructions

This test may exhibit interference when sample is collected from a person who is consuming a supplement with a high dose of biotin (also termed as vitamin B7 or B8, vitamin H, or coenzyme R). 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.

Expected Turnaround Time

Within 1 day

Related Documents

For more information, please view the literature below.

Thyroid Testing: Assessing Thyroid Disease in Your Patients

Specimen Requirements


Serum (preferred) or plasma


1 mL (adult), 0.8 mL (pediatric)

Minimum Volume

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


Red-top tube, gel-barrier tube, or green-top (lithium heparin) tube. Do not use oxalate, EDTA, or citrate plasma.


If a red-top tube or plasma is used, transfer separated serum or plasma to a plastic transport tube.

Storage Instructions

Room temperature

Stability Requirements



Room temperature

14 days


14 days


14 days

Freeze/thaw cycles

Stable x3

Test Details


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.


T4 may be increased with excess intake of iodine or with surreptitious use of thyroxine.3 T4 levels may be abnormal in the presence of systemic nonthyroidal disease. Alterations in binding capacity or quantity of TBG may increase or decrease total thyroxine without causing symptoms. A common cause of elevated T4 in nonthyroidal disease is said to be liver disease. Serum thyroxine and free thyroxine index (FTI) are increased in familial dysalbuminemic hyperthyroxinemia, a euthyroid syndrome in which an abnormal binding site has affinity for thyroxine.4 The T3 is usually normal in this entity, as is T3 uptake (THBR). Thus, T3 uptake is commonly ordered with T4.


Electrochemiluminescence Immunoassay (ECLIA)

Reference Interval

4.5−12.0 μg/dL1,2

Additional 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.5,6 High dose propranolol may elevate T4 and FTI levels.7

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.


1. Soldin SJ, Hicks JM, eds. Pediatric Reference Ranges. Washington, DC: AACC Press;1995:141.
2. Murthy JN, Hicks JM, Soldin SJ. Evaluation of the technicon Immuno I random access immunoassay analyzer and calculation of pediatric reference ranges for endocrine tests, T-uptake, and ferritin. Clin Biochem. 1995 Apr; 28(2):181-185. 7628078
3. Klein I, Levey GS. Silent thyrotoxic thyroiditis. Ann Intern Med. 1982 Feb; 96(2):242-244. 7059071
4. Ruiz M, Rajatanavin R, Young RA, et al. Familial dysalbuminemic hyperthyroxinemia: A syndrome that can be confused with thyrotoxicosis. N Engl J Med. 1982 Mar 18; 306(11):635-639. 6173750
5. Borst GC, Eil C, Burman KD. Euthyroid hyperthyroxinemia. Ann Intern Med. 1983 Mar; 98(3):366-378. 6187257
6. Chopra IJ, Hershman JM, Pardridge WM, Nicoloff JT. Thyroid function in nonthyroidal illnesses. Ann Intern Med. 1983 Jun, 98(6):946-957 (review). 6407376
7. Cooper DS, Daniels GH, Ladenson PW, Ridgway EC. Hyperthyroxinemia in patients treated with high-dose propranolol. Am J Med. 1982 Dec; 73(6):867-871.6816067


Franklyn JA, Davis JR, Ramsden DB, Sheppard MC. Phenytoin and thyroid hormone action. J Endocrinol. 1985 Feb; 104(2):201-204. 3918137
Gharib H, Klee GG. Familial euthyroid hyperthyroxinemia secondary to pituitary and peripheral resistance to thyroid hormones. Mayo Clin Proc. 1985 Jan; 60(1):9-15. 2981377
Gruhn JG, Barsano CP, Kumar Y. The development of tests of thyroid function. Arch Pathol Lab Med. 1987 Jan; 111(1):84-100. 3541847
Surks MI, Chopra IJ, Mariash CN, Nicoloff JT, Solomon DH. American Thyroid Association guidelines for the use of laboratory tests in thyroid disorders. JAMA. 1990 Mar 16, 263(11):1529-1532. 2308185


Order Code Order Code Name Order Loinc Result Code Result Code Name UofM Result LOINC
001149 Thyroxine (T4) 3026-2 001149 Thyroxine (T4) ug/dL 3026-2

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