Total Thyroxine (TT4): Optimal Testing Recommendations

  • Thyroid-stimulating hormone (TSH) with reflex to free Thyroxine (FT4), the unbound active form of thyroxine is recommended instead of TT4 to assess thyroid function and diagnose disease in nonpregnant, asymptomatic adults.
  • For pregnant women, if initial TSH is abnormal, measurement of TT4 (with a pregnancy-adjusted reference range) instead of free T4 is recommended for estimating hormone levels during last part of pregnancy.

Guidelines for Test Utilization

What does the test tell me?

The test informs how well the thyroid is functioning - euthyroidism, hypothyroidism or hyperthyroidism. There are two forms of thyroxine that can be measured:

  • TT4 which includes measurement of both thyroxine that is bound to protein and thyroxine that is not bound to protein;
  • Free T4 which is not bound to protein and is the active form that can enter various tissues and exert effects. Free T4 is regarded as the most accurate test to assess thyroid function. [back to top]

When should I order this test?

If clinical symptoms suggest a primary disorder of hypothalamic–pituitary–thyroid function.

  • A decrease or increase in metabolism described as unexplained weight gain or loss, depression, anxiety, changes in menstrual cycle, palpitations, subfertility, focal thyroid nodule heat or cold intolerance, etc.
  • A family history of autoimmune thyroiditis
  • Goiter is detected on physical exam
  • Monitoring treatment with synthetic triiodothyronine, which can cause a low TT4.Monitoring treatment of hyperthyroidism with anti-thyroid drugs.
  • Evaluating thyroid function when the thyroxine-binding globulin (TBG) is normal and non-thyroidal illness is not present.

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When should I NOT order this test?

It is not recommended to use TT4 to screen for thyroid dysfunction in non-pregnant, asymptomatic adults. [back to top]

How should I interpret the result?

Use FT4 to further interpret TT4 findings instead of TT4 alone.

  • Elevated TSH and low FT4 indicates primary hypothyroidism - thyroid gland disease
  • Elevated TSH and normal FT4 - suggests subclinical hypothyroidism, euthyroid sick syndrome, transient thyroiditis, autoimmune thyroiditis which needs further workup by measuring thyroid antibodies and total triiodothyronine (TT3)
  • Elevated TSH and high FT4 - suggests secondary hyperthyroidism
  • Low TSH and low FT4 - indicates secondary hypothyroidism.
  • Low TSH and an elevated FT4 - indicates hyperthyroidism.

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Is the test result diagnostic/confirmatory of the condition?

A TT4 result is not diagnostic and should be interpreted within the context of TSH, FT4 and free triiodothyronine (FT3) concentrations to further evaluate the thyroid function. [back to top]

Are there factors that can affect the lab result?

  • Non-thyroidal illness* can impact TT4 results. * e.g., acute/chronic illness - poor nutrition/starvation, sepsis, renal disease, and recent treatment for thyrotoxicosis (medications such as thyroxine, amiodarone, lithium)
  • Patients who use dienogest and levonorgestrel low dose containing oral birth control pills may have increased TT4 due to an elevated binding to serum thyroid-binding globulins, while the free proportion of T4 may slightly be affected.
  • Heterophile antibodies and human anti-mouse antibodies (HAMA) that are present in specimens can interfere with accurate results (false negative or false positive results can occur).
  • Exogenous biotin with high concentrations can interfere with the accurate measurement of T4 if the methodology employs biotin as a component of the reagent.

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Are there considerations for special populations?

During the first trimester of pregnancy estrogen-mediated increases in circulating levels of thyroid-binding globulin can increase TT4 and lower the FT4 circulating concentrations. FT4 may be lower in pregnancy than non-pregnancy. Current guideline (2017) suggests, instead of measuring FT4, measurement of TT4 (with a pregnancy-adjusted reference range) is a reliable proxy to estimate hormone concentrations during last part of pregnancy. [back to top]

What other test(s) might be indicated?

TSH, FT4 and free triiodothyronine (FT3) test may be used to further evaluate thyroid function. [back to top]

References

Koulouri Oet al. Pitfalls in the measurement and interpretation of thyroid function tests. Best Pract Res Clin Endocrinol Metab. 2013;27(6):745-762. 

Screening for thyroid dysfunction: U.S. Preventive Services Task Force recommendation statement. Ann Intern Med. 2015;162(9):641–650.

Alexander EK, et al. 2017 Guidelines of the American Thyroid Association for the Diagnosis and Management of Thyroid Disease During Pregnancy and the Postpartum. Thyroid 2017; 27:315-430.

Wiegratz I et al. Effect of four oral contraceptives on thyroid hormones, adrenal and blood pressure parameters. Contraception. 2003 May;67(5):361-6. 


Last reviewed: February 2022. The content for Optimal Testing: the Association for Diagnostics & Laboratory Medicine’s (ADLM) Guide to Lab Test Utilization has been developed and approved by the the Academy of Diagnostics & Laboratory Medicine and ADLM’s Science and Practice Core Committee.

As the fields of laboratory medicine and diagnostic testing continue to grow at an incredible rate, the knowledge and expertise of clinical laboratory professionals is essential to ensure that patients received the highest quality and most useful laboratory tests. ADLM’s Academy and Science and Practice Core Committee have developed a test utilization resource focusing on commonly misused tests in hospitals and clinics. Improper test utilization can result in poor patient outcomes and waste in the healthcare system. This important resource geared toward medical professionals recommends better tests and diagnostic practices. Always consult your laboratory director to make sure these recommendations are appropriate for your patient population.