Understanding A Thyroid Function Test

The thyroid is a small, butterfly-shaped endocrine gland located in the front of the neck. It is crucial in managing several bodily functions and often in autoimmune disease, poor lifestyle, or lack of iodine it might become dysfunctional either stimulating too much or too little thyroid hormones. A thyroid function test is recommended when your doctor might suspect your thyroid is not functioning well. The three main tests for thyroid disorders are TSH, T3 and T4. Sometimes, to make a better diagnosis, a doctor will also suggest a FT3 and FT4 test and Anti TPO test as well.


Your hypothalamus releases thyrotropin-releasing hormone (TRH) triggering the pituitary gland to release thyroid-stimulating hormone (TSH). TSH then stimulates the thyroid to release T3 and T4. It is TRH which is the master regulator of thyroid gland growth and function and together with T3 and T4 contribute to various functions like metabolic rate, heat generation, neuromuscular function, and heart rate and others.

The Thyroid gland makes mostly T4 and just a little T3. Most of the T3 needed by the body is converted from T4 and this happens outside the thyroid namely in the liver, but also the heart, muscle, gut, and nerves. Helping this conversion are two enzymes called deiodinases. It is T3 that is the active thyroid hormone which with some help from T4 influences bodily function.

TSH: Thyroid Stimulating Hormone

TSH levels in the blood are often used as a marker to assess the functioning of the thyroid gland. When the thyroid gland is not producing enough thyroid hormones (a condition known as hypothyroidism), the pituitary gland increases its production of TSH to try to stimulate the thyroid to produce more hormones. Conversely, when the thyroid gland is overactive and producing too many hormones (a condition known as hyperthyroidism), the pituitary gland decreases its production of TSH.

Measuring TSH levels through from blood in a thyroid function test is a common way to diagnose thyroid disorders and to monitor the effectiveness of thyroid treatment. Abnormal TSH levels can indicate thyroid dysfunction, and healthcare providers use these results to guide treatment decisions for thyroid conditions.

NORMAL LEVELS OF TSH: 0.4-4.0 mIU/L (milli-international units per liter). However, a more perfect level would be between 0.5 and 2.5 mIU/L

HIGH TSH: The pituitary gland is secreting more thyroid stimulating hormone for an underactive thyroid to produce more thyroid hormones. This occurs in hypothyroidism.

LOW TSH: The pituitary gland is secreting less thyroid stimulating hormone for an overactive thyroid which needs to produce fewer thyroid hormones. This occurs in hyperthyroidism

Sometimes TSH can show up as zero which means an excess of thyroid hormone in your body. Sometimes hyperthyroidism influenced by its causes might not usually respond to the TSH signal in which case medication is needed to block hormone production.

T4 & FT4

A Total T4 measures the bound and Free Thyroxine (T4) hormone levels in your blood. A Free T4 measures what is not bound and able to freely enter and affect the body tissues.  In usual cases to rule out thyroid disorder, a T4 with  FT4 test provides a better diagnosis. T4 and T3 Levels are usually read in relation to TSH.

NORMAL LEVELS OF T4: 5.0 to 12.0 μg/dL.

NORMAL LEVELS OF FT4: 0.8 to 1.8 ng/dL.


A Total T3 test measures the bound and free fractions of triiodothyronine. T3 tests are mostly recommended to diagnose hyperthyroidism and can determine the severity.

High T3 and Low TSH: Hyperthyroidism

In some cases of hyperthyroidism with low TSH, only T3 is elevated and T4 or FT4 remains normal. While low T3 can indicate hypothyroidism, T3 is not helpful in diagnosing it because it is the last test to become abnormal and levels of T3  fall late during hypothyroidism.

  • Normal T3 Level: 60 to 180 ng/dL. FT3:
  • Normal FT3 Levels:2.3 – 4.1 pg/mL.

Thyroid Peroxidase Antibodies

Besides the TSH, T3 and T4 in a thyroid function test, a doctor may also recommend a Thyroid antibodies test. TPO or Thyroid peroxidase is an enzyme in the thyroid responsible for thyroid hormone production. In autoimmune conditions, lymphocytes react and attack the thyroid producing antibodies against the thyroid enzyme thinking it a bad guy.

The presence of TPO antibodies in the blood can indicate Hashimoto’s thyroiditis. In Hashimoto’s thyroiditis, TPO antibodies target and damage the thyroid gland, leading to inflammation and, over time, reduced thyroid function (hypothyroidism). Elevated TPO antibody levels can help confirm the diagnosis of Hashimoto’s thyroiditis.

Normal Antibodies Level: less than 34 (IU/ml).

Antibodies: What do they indicate?

  • TPOAb (Thyroid peroxidase antibodies): Hashimoto’s thyroiditis, sometimes Graves’ disease
  • Tg Ab (Thyroglobulin antibodies: Hashimoto’s thyroiditis
  • TSHR Ab (Thyroid stimulating hormone receptor antibodies): Graves’ disease

When Is the Best Time to Take a Thyroid Function Test

The timing of a thyroid function test can affect the results, so it’s generally recommended to have the test done in the morning and under certain conditions for the most accurate and consistent results.

  • Morning Testing: The best time to have a thyroid function test is typically in the morning, as thyroid hormone levels can naturally fluctuate throughout the day. TSH levels tend to be at their highest in the early morning and may be lower in the afternoon or evening. Morning testing helps ensure that you get the most accurate TSH measurement.
  • Fasting: It is often recommended to have the test done after an overnight fast, meaning you should not eat anything for about 8 to 12 hours before the blood draw. This helps eliminate any potential interference from food or drink on the test results, especially when measuring cholesterol or triglyceride levels in addition to thyroid function.

Abnormal Test Results Not Due to Thyroid Disorder

The availability of blood tests to measure thyroid hormones and thyroid stimulating hormone (TSH) is widespread, but it’s important to keep in mind that not all tests are appropriate in every situation and that the results of thyroid tests can be impacted by a variety of factors, including medications, dietary supplements, and conditions unrelated to the thyroid. When there is a mismatch between your thyroid test results and how you feel, an endocrinologist can assist you in understanding the data. Repeating the test is frequently a useful initial step to make sure there are no drugs present that might affect the test findings. The factors that can impact a thyroid test are:

•          Pregnancy

•          Medications

•          Illness or infection

•          Fasting and meal timings

•          Stress, surgery, or trauma

•          Medical conditions

•          Age

•          Lab conditions

•          Dietary supplements like b vitamins

There is no preparation needed to take a thyroid function test although it is usually recommended in the morning which provides the best results. Furthermore, you need to tell your doctor if you are on any medications that might affect your test results.