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Thyroxine (T4)

Code:9002|CPT:84436|LOINC:3026-2

Synonyms
Общий Т4тетрайодтиронинFree T4T4TetraiodothyronineThyroxineTotal T4Total tetraiodothyronine
IncludesThyroxine (T4)

Analysis details

Methodology

  • Electrochemiluminescence immunoassay (ECLIA)
  • Chemiluminescent immunoassay (CLIA)

Expected Turnaround Time

1 day

Special Instructions

  • Avoid high‑dose biotin (vitamin B7/B8, vitamin H, coenzyme R) for at least 72 hours before collection and inform the ordering clinician about any biotin use.
  • Do not eat for 2–3 hours before the blood draw; water is allowed.
  • With clinician approval, hold steroid and thyroid hormone medications for 48 hours prior to collection.
  • Avoid vigorous exercise and significant emotional stress for 24 hours before collection.
  • Do not smoke during the 3 hours preceding collection.

How to use

The Total Thyroxine (T4) test is used to assess thyroid function and to diagnose and monitor thyroid disorders. Values decrease in hypothyroidism and in the third stage of painful subacute thyroiditis, and increase in hyperthyroidism, in the initial phase of subacute thyroiditis, and in thyrotoxicosis associated with Hashimoto disease. Results are typically interpreted with TSH and, when indicated, complementary thyroid tests. Total T4 (tetraiodothyronine) also contributes to the evaluation of female infertility related to thyroid dysfunction and is used in screening for congenital hypothyroidism. It is applied for therapeutic monitoring in patients receiving treatment for thyroid disease.

Limitations

Total T4 reports the combined protein‑bound and free fractions of thyroxine, a principal hormone governing basal metabolic rate and energy utilization. The measured concentration depends on serum binding proteins—especially thyroxine‑binding globulin (TBG) and albumin—so changes in binding capacity or quantity can alter total T4 without a corresponding shift in the biologically active free fraction. Total T4 can be abnormal in systemic nonthyroidal illness. Familial dysalbuminemic hyperthyroxinemia produces elevated total T4 and a high free thyroxine index with normal T3 and T3 uptake in clinically euthyroid individuals; T3 uptake is often ordered with T4 to aid interpretation. Total T4 varies by phase in subacute thyroiditis. Excess iodine intake and exogenous thyroxine can increase total T4 concentrations.

Unitmcg/dL
Reference interval
AgeMinMax
≤3d4.812.7
IndicationsWorkup of suspected hyperthyroidism with symptoms such as tachycardia, irritability, weight loss, insomnia, tremor, diarrhea, photophobia or visual disturbance, periorbital edema, ocular dryness or erythema, or exophthalmos, Assessment of possible hypothyroidism presenting with weight gain, xerosis, constipation, cold intolerance, edema, alopecia, or menstrual irregularity; severe disease may lead to arrhythmia, myocardial ischemia, or myxedema coma; in children, risk of developmental delay/cretinism, Preventive screening of thyroid function in conjunction with other laboratory studies, Periodic monitoring of thyroid disease therapy together with TSH (at least every 3 months), Evaluation during pregnancy in individuals with thyroid disease or risk factors for thyroid dysfunction, Neonatal testing in the first days of life for infants born to mothers with thyroid disease

Possible Causes of Abnormal Results

Increased levels

  • amiodarone
  • aspirin
  • biotin
  • danazol
  • familial dysalbuminemic hyperthyroxinemia
  • furosemide
  • iodine (excess intake)
  • levothyroxine
  • liver disease
  • propranolol
  • propylthiouracil
  • tamoxifen
  • valproic acid

Decreased levels

  • decreased tbg
  • nonthyroidal illness

Specimen Requirements

SpecimenSerum
ContainerGold/Tiger Top (SST, Gel Separator)
Volume1 mL (min 0.7 mL)
Storage InstructionsRoom temperature, Refrigerated, Frozen

References

Franklyn JA, Davis JR, Ramsden DB, Sheppard MC. Phenytoin and thyroid hormone action. J Endocrinol. 1985 Feb;104(2):201-204. PubMed 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. PubMed 2981377

Gruhn JG, Barsano CP, Kumar Y. The development of tests of thyroid function. Arch Pathol Lab Med. 1987 Jan;111(1):84-100. PubMed 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. PubMed 2308185