Part of the endocrine system, the thyroid gland plays a major role in several body processes.

It governs growth, development, and metabolism through the hormones it releases.

The two proper hormones produced in the thyroid gland, as you might be familiar with, are T3 or Triiodothyronine and T4 or Tetraiodothyronine / Thyroxin.

They are produced in the follicular epithelial cells with the help of iodine, a trace element found in seafood, kelp, and organic potatoes, to name a few.

When these hormones are under or over its usual levels, you can develop hypothyroidism, hyperthyroidism, and other thyroid-related maladies.

If the doctor suspects you to suffer from a thyroid problem, or if he wants to exclude it from his diagnosis, he will ask you to undergo thyroid tests.

These exams will check the levels of the several hormones in your blood stream. As such, it is important that you learn more about the normal and optimal levels of the following thyroid hormones in your body:

Free T4 / Free Thyroxine / FT4

Proteins attach to the hormone T4. Known as bound T4, these hormones are stored in the bloodstream and released only as and when the body needs it.

However, there are T4 hormones which are not bound to these proteins, and they are aptly called Free T4. These hormones can be utilized by tissues and organs immediately.

The Free T4 test is preferred by most physicians due to the fact that some proteins can affect the levels of Bound T4 in the body. If the levels of FT4 and the Thyroid-Stimulating Hormone test are both abnormal, this will help confirm the occurrence of a thyroid disorder.

The normal and optimal range of FT4 is 0.9 to 1.2 nanograms per deciliter, in adults aged 20 years and above. In the pediatric population, the normal levels are as follows:

If your levels go above the normal range of FT4 in the body, your physician might consider the following diagnoses:

• Graves Disease
• Thyroiditis
• Toxic Goiter
• Toxic Thyroid Nodules
• Ovarian or Testicular Cancer, albeit rare

Increased FT4 is not only associated with the above illnesses, other causes include consumption of Iodine-rich foods and excessive intake of thyroid medications.

This might also occur if you have an underlying thyroid problem and have undergone a diagnostic test which utilizes a contrast dye.

Should your results yield lower than the normal FT4 levels, your doctor will diagnose you with Hypothyroidism (i.e. Hashimoto’s Disease.) This is upon exclusion of the other causes of low FT4 levels, which occurs in the state of severe acute illness, malnutrition or fasting, or with the intake of various medications.

Deranges in FT4 levels can also occur with dysalbuminemic hyperthyroxinemia, immunotherapy, and procedures utilizing immunoglobulins or its fragments.

Total T4 / Serum Thyroxine

As it has been mentioned, there are two types of T4 hormones in the body: free and bound. Both of these hormones are measured in the Total T4 (TT4) test. Physicians employ this blood exam to check the patient’s response to hyperthyroidism treatment.

The normal and optimal values of TT4 in adults aged 20 years and above are 4.5 to 11.7 micrograms per deciliter. In pediatric patients, the normal measurements are the following:

A decrease in TT4 levels comes with the following conditions:

• Hypothyroidism
• Myxedema
• Cretinism
• Chronic or Subacute Thyroiditis

Low TT4 levels are also common in patients being treated with anabolic steroids or in those suffering from nephrosis.

An increase in TT4 levels often signifies hyperthyroidism or acute thyroiditis. However, pregnant women or those taking estrogen medications also exhibit high TT4 levels.

Hyperthyroidism in pregnant women is only definitive following an increase in TT4 and resin T3 uptake, according to Soldin in his 2006 study.

In light of that, diagnoses with TT4 only are not definitive as your physician will need to correlate it with the results of your other thyroid function tests.

Free T3 (FT3)

This exam is prescribed by the doctor to determine the patient’s true thyroid status, as biologically-active FT3 hormones represent a mere 0.5% of the body’s total T3 hormones. It confirms diagnosis of hyperthyroidism in conjunction with T4 and Total T3 assays. It also helps monitor an individual’s response to thyroid hormone replacement.

The normal and optimal levels of FT3 are 2.8 to 4.4 picograms per milliliter. Elevations in such foretell FT3 thyrotoxicosis especially if they are seen with normal FT4 and TT3, and subnormal levels of TSH, according to a study by Figge et al. Too much intake of thyroid hormone replacement can also increase FT3 levels in the body.

Total T3 / Serum Triiodothyronine

The TT3 exam is requested by doctors to confirm hyperthyroidism in patients with normal thyroxine and low TSH levels. It is also employed in the diagnosis of triiodothyronine toxicosis. The normal and optimal levels for people aged 20 years old and above are 80 to 200 nanograms per deciliter. As for children and young adults, the appropriate ranges are as follows:

Elevations in T3 and T4 levels confirm hyperthyroidism. However, a TT3 exam is deemed to be a better test for diagnosing hyperthyroidism as T3 levels increase before T4 levels rise.

There are also hyperthyroid individuals who only have TT3 elevations, normal T4, and TSH suppression, as is the case of T3 Thyroxicosis.

Low levels of both T3 and T4 occur in hypothyroidism, although sick or euthyroid patients admitted in hospitals also exhibit decreased T3 levels.

Starvation, as well as intake of Inderal, steroids, amiodarone, salicylates, phenytoin and phenylbutazone can also lead to low TT3 levels.

Thyroid Stimulating Hormone (TSH)

A good indicator of thyroid status, this exam determines the levels of TSH in the body. TSH is important in thyroid function as it stimulates the gland’s growth as well as the production of hormones.

The TSH exam is also done for the following purposes:

• monitor the patient’s response to hormone replacement therapy
• confirm suppression of TSH release in thyroid cancer patients receiving thyroxine suppressive treatments
• predict the onset of thyrotropin-releasing hormone-stimulated TSH response

For adults aged 20 years old and above, the normal TSH levels are 0.3 to 4.2 milli-international units per liter. In children, the optimal ranges are as follows:

Increased levels of TSH in the body oftentimes determine hypothyroidism. This is due to the pituitary gland’s efforts of releasing TSH in your blood in order to cope up with your underactive thyroid gland.

TSH levels are important for distinguishing primary from secondary and tertiary hypothyroidism. Rates are increased in primary hypothyroidism, while the concentration is normal to low in the latter two.

Decreased levels of TSH, on the other hand, diagnose hyperthyroidism. Since the thyroid gland produces an excess of hormones, the pituitary gland compensates by stopping the creation and release of TSH in the body.

Do note that transient or falsely low TSH levels can occur in individuals who are sick or hospitalized.

Abnormal TSH results will oftentimes necessitate other thyroid exams, such as the ones stated above, to accurately confirm the diagnosis of hyper- or hypothyroidism.

Anti-Thyroid Antibodies / Autoantibodies (TAB)

Two antibodies are checked in this exam, namely the thyroglobulin and thyroperoxidase antibodies. The normal values are <4.0 and <9.0 international units for these antibodies respectively.

Thyroglobulin antibodies, which are pivotal in the storage, release, and synthesis of thyroid hormones, are normally not seen in the circulation. They are leaked, however, in the occurrence of any of the following conditions: Graves disease, Hashimoto’s disease, neonatal hypothyroidism, postpartum thyroiditis, and autoimmune thyroid illnesses. The TAB exam is also used to measure the degree of treatment in individuals with follicular cell-derived thyroid carcinomas.

Thyroperoxidase (TPO) antibodies are also essential for thyroid hormone synthesis, and are activated following thyroid dysfunction. It is utilized to distinguish thyroid autoimmune conditions from hypothyroidism or non-autoimmune goiter. It is also used to determine the need for treatment of a patient manifesting signs and symptoms of subclinical hypothyroidism.

According to a 2010 study by Zelaya et al, TPO measurement is also beneficial for individuals with normal to high TSH levels, as they can be at risk of developing hypothyroidism in the future.

Thyroid Binding Globulin (TBG)

In persons with no signs or symptoms of thyroid dysfunction but with low levels of thyroid hormone levels, the TBG exam is warranted. The normal and optimal levels are 12-26 micrograms per milliliter in men, and 11 to 27 micrograms per milliliter in women.

Elevated levels are found in persons who are taking contraceptive steroids/estrogen or women who are pregnant. Increases in TBG can go as high as 2 to 3 fold with estrogen, according to a study by Ain et al.

Low levels, on the other hand, occur with ailments such as liver disease, nephrotic syndrome and hypoproteinemia. It also occurs in people who use glucocorticoids, androgenic, or anabolic steroids excessively.

Thyroglobulin (TG)

TG determines thyroid tissue function. It is recommended in individuals with thyroid cancer patients to see if there is residual thyroid tissue, or if the cancer has recurred or metastasized following surgery or radiotherapy.

In patients with intact thyroid tissues, the normal levels should be below or equal to 33 nanograms per milliliter. For those devoid of thyroid tissue, the levels should be below 0.1 nanograms per milliliter. For these people, a finding of 10 nanograms per milliliter or more signify residual or recurring disease.

More Info Regarding Thyroid Function Tests

As with other laboratory exams, thyroid function tests involve the extraction of blood; approximately 0.5 to 1 ml of blood will be drawn from your arm. While it is a routine procedure, it still comes with minor risks, such as bleeding or lightheadedness. In some cases, an infection or a hematoma might develop following the blood extraction process.

Should your doctor ask you to undergo FT3, TT3, FT4, TT4, TAB, or TG testing, you need to avoid taking pills containing Vitamin B7 or Biotin 12 hours prior to the procedure.

You need to inform your physician if you are taking dextrothyroxine or other drugs that lower lipid levels in the body. Such medications can interfere with the results. You will need to stop taking these drugs for 4 to 6 weeks in order to obtain accurate FT4 or TT4 results.

Exposure to animal antigens and thyroid hormone autoantibodies can also yield false FT3, TT3, TT4, or TSH results. Again, inform your physician about your clinical history so that accurate thyroid hormone measurements can be done.

References:

Ain, K. B., Refetoff, S., Same, D. H., & Murata, Y. (1988). Effect of estrogen on the synthesis and secretion of thyroxine-binding globulin by a human hepatoma cell line, Hep G2. Journal of Molecular Endocrinology,2(4), 313-323. Retrieved August 30, 2017, from https://www.ncbi.nlm.nih.gov/pubmed/2837662.

Burch, H., MD. (2017, May). Thyroid Tests. Retrieved August 30, 2017, from https://www.niddk.nih.gov/health-information/diagnostic-tests/thyroid

Figge, J., Leinung, M., Goodman, A. D., Izquierdo, R., Mydosh, T., Gates, S., Line, B., Lee, D. W. (1994). The clinical evaluation of patients with subclinical hyperthyroidism and free triiodothyronine (free T3) toxicosis. [Abstract]. American Journal of Medicine, 96(3), 229-234. Retrieved August 30, 2017, from https://www.ncbi.nlm.nih.gov/pubmed/8154510.

How does the thyroid work? (2015, January 7). Retrieved August 30, 2017, from https://www.ncbi.nlm.nih.gov/pubmedhealth/PMH0072572/

Soldin, O. P. (2006). Thyroid Function Testing in Pregnancy and Thyroid Disease: Trimester-specific Reference Intervals. Therapeutic Drug Monitoring, 28(1), 8–11.

T3 (Triiodothyronine), Free, Serum. (2017). Retrieved August 30, 2017, from http://www.mayomedicallaboratories.com/test-catalog/Specimen/9404

Thyroglobulin, Tumor Marker, Serum. (2017). Retrieved August 30, 2017, from http://www.mayomedicallaboratories.com/test-catalog/Clinical and Interpretive/62800

Thyroid Function Tests. (2017). Retrieved August 30, 2017, from https://www.auburn.edu/~deruija/endo_thyroidfts.pdf

Thyroid-Stimulating Hormone-Sensitive (s-TSH), Serum. (2017). Retrieved August 30, 2017, from http://www.mayomedicallaboratories.com/test-catalog/Clinical and Interpretive/8939

Thyroxine-Binding Globulin (TBG), Serum. (2017). Retrieved August 30, 2017, from http://www.mayomedicallaboratories.com/test-catalog/Clinical and Interpretive/9263

Wisse, B. (2016, February 3). Free T4 Test. Retrieved August 30, 2017, from  https://medlineplus.gov/ency/article/003517.htm

Zelaya , A. S., Stotts, A., Nader, S., & Moreno, C. A. (2010). Antithyroid peroxidase antibodies in patients with high normal range thyroid stimulating hormone. [Abstract]. Journal of Family Medicine, 42(2), 111-115. Retrieved August 30, 2017, from https://www.ncbi.nlm.nih.gov/pubmed/20135568.