Thyroid Function Tests: Purpsose, Procedure and Understanding the Levels

Thyroid function tests are a series of blood tests which include T3, T4, and TSH and are done to determine how well your thyroid gland is working.

Updated: June 16, 2022

Thyroid function tests are a series of blood tests which include T3, T4, and TSH and are done to determine how well your thyroid gland is working.
Thyroid is a small butterfly shaped gland that is located in the lower-front part of your neck that typically produces three hormones namely triiodothyronine (T3), thyroxine (T4), and calcitonin.
The thyroid produces hormones when the pituitary gland releases TSH in the blood. Thyroid-stimulating hormone or TSH is responsible for regulation of metabolism, release of hormones by the thyroid gland, energy generation, and mood. A TSH blood test to check for the existence of any underlying causes of a typical production of hormones in these two glands. If the TSH level comes abnormal, then further screening for overactive or underactive thyroid gland is done.

Purpose of TFT:

Thyroid function tests are done to

  • determine whether or not the thyroid gland is working properly. Some of the symptoms of an underactive thyroid gland or hypothyroidism are tiredness, dry skin, weight gain, constipation, a feeling of being too cold, or frequent menstrual periods.
  • signs of an overactive thyroid or hyperthyroidism are nervousness, diarrhea, weight loss, rapid heart rate, feeling too hot, or experiencing irregular menstrual periods.
  • adjust the dose of thyroid medications.
  • look into the cause of hypothyroidism. TSH level in the bloodstream can help determining if hypothyroidism is caused because of a damaged thyroid gland or some other causes, such as hypothalamus, etc.
  • track the functionality of the thyroid gland in case of hyperthyroidism. This may include surgery, antithyroid medicine prescription, or radiation therapy.
  • diagnose congenital hypothyroidism which occurs in case of a newborn baby.

Cause of Fluctuation in TSH:

The thyroid gland is regulated by the pituitary gland and the pituitary is regulated in part by the thyroid gland along with the hypothalamus. The hypothalamus releases a hormone called thyrotropin releasing hormone (TRH), which sends a signal to the pituitary to release thyroid stimulating hormone (TSH). TSH sends a signal to the thyroid to release thyroid hormones T3 and T4. TSH will be released in more quantity to stimulate the thyroid when T3 and T4 levels are low. But when T3 & T4 levels are high, less quantities of these two hormones are produced. If a disruption occurs at any of these levels, a defect in thyroid hormone production may result in a deficiency of thyroid hormone (hypothyroidism).
In persons with hypothyroidism, thyroid hormone production is below normal and there is a continuously decreased level of circulating thyroid hormones. In persons with hyperthyroidism, thyroid hormone production is above normal and there is a continuously elevated level of circulating thyroid hormones.
TSH levels are also influenced by several other factors, such as

  • Genetics
  • Poisonous substances and radiation exposure
  • Inflammation of the thyroid gland
  • Pregnancy
  • Thyroid cancer
  • Deficiency or excess of iodine in the diet
  • Certain medications such as antidepressants, cholesterol lowering drugs, chemotherapy drugs, steroids

Procedure for TFT:

A band will be tied by a technician around your arm so that the veins of the hand are more prominent to prick the injection. The vein is punctured with a fresh, disposable syringe and the blood is withdrawn after the site is cleaned with an alcohol swab.
A cotton swab will be given to you after drawing the blood to place on the place where the injection was pricked. You will be asked to apply pressure on the cotton in order to stop the bleeding. You might notice a bruise in the place where the injection was pricked on your skin. This is normal and should fade away in a few days. But if you experience unbearable pain, or if the area around the puncture becomes red and swollen, immediately follow up with your doctor. These could be signs of an infection. Before you undergo this test, you should inform the the lab technician if you suffer from a bleeding or clotting disorder or are taking medicines like aspirin, warfarin or other blood-thinning agents, as in such cases the bleeding might take a while to stop. As certain medications and being pregnant may influence your test results, inform your doctor about these things before the test.

Understing the TSH Levels:

Normal TSH levels:

The normal TSH levels in an average adult range between 0.4 and 4.0 mIU/L (milli-international units per liter). TSH level should be between 0.5 and 2.5 mU/L, for people who are already taking thyroxine.
The reference ranges varies slightly as we grow older and in the case of a pregnancy. In premature babies fo 28 to 36 weeks, The TSH level should be 7 to 27 mIU/L.
TSH level should be 1 39 mIU/L in children of birth to 4 days. In 2 to 20 weeks it can be 1.7 to 1 mIU/L and in 21 weeks to 20 years it should be 0.7 to 64 mIU/L.
In case of adults of age 21 to 54 years it should be 0.4 to 2 mIU/L where as during age of 55 to 87 years it can be 0.5 to 9 mIU/L.
TSH level during pregnancy should be 0.3 to 5 mIU/L, 0.3 to 6 mIU/L and 0.8 to 2 mIU/L in 1st, 2nd and 3rd trimestar respectively.
The ranges may vary from one laboratory to another due to the use of different measurement techniques. The best way to find the accurate levels is by consulting a physician.

High TSH Levels:

When TSH level is greater then 4.0 mIU/L, it is an indication of an underactive thyroid gland or the condition of hypothyroidism where the producion of thyroid hormone is very less. Radiation treatment, an autoimmune disease known as Hashimoto's disease, or surgical removal of the thyroid gland are some of the common causes of hypothyroidism.

Low TSH levels:

When TSH reading is below 0.4 mIU/L, it indicate an overactive thyroid gland, otherwise known as a condition of hyperthyroidism where the thyroid gland produces more than required quantities of T3 and T4 hormones. Hyperthyroidism is also closely associated with goiter, Grave's disease, excessive content of iodine in the body, or an overdose of synthetic thyroid hormone.
If the blood results are abnormal, an ultrasound test is suggested which will check for structural problems with the thyroid gland, thyroid gland activity, and any tumors that may be causing problems.
Based on these findings, sample tissue from the thyroid can be checked for cancer. If the scan is normal, your thyroid function can be regulated by prescribed medication. The doctor will follow up with additional thyroid function tests to make sure the medication is working properly.
Comprehensive screening of thyroid health should ideally include these 6 tests:

  • TSH
  • Free T3
  • Free T4
  • Reverse T3
  • Thyroid Peroxidase Antibodies
  • Thyroglobulin Antibodies


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