What Is the Thyroid Gland?
An endocrine gland (glands that secrete hormones) located in the front of the neck, on the lower part, is called the thyroid gland. It is a butterfly-shaped gland that secretes thyroid hormones. The secreted thyroid hormones enter the bloodstream, from where it reaches every body tissue. These thyroid hormones are needed to maintain average body temperature and the normal functioning of the heart, muscles, brain, and other organs.
What Are the Hormones Secreted by the Thyroid Gland?
Utilizing the Iodine that is taken through food, the thyroid gland secretes two hormones, namely,
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Triiodothyronine (T3).
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Thyroxine (T4).
Thyroxine contains four Iodine atoms, of which one is removed in the liver and brain to become Triiodothyronine.
What Is the Mechanism of the Release of Thyroid Hormones?
The thyroid gland secretes not only these hormones but also stores and releases them when the need arises. The need to release thyroid hormone is determined by the hypothalamus (a small but significant portion of the brain) and pituitary gland (an endocrine gland located in the skull base and is the size of a pea). The hypothalamus releases Thyrotropin-releasing hormone (TRH), and the pituitary gland releases Thyroid-stimulating hormone (TSH).
When the thyroid hormone level is low, the hypothalamus senses it. It releases TRH, which stimulates the pituitary gland to release TSH, which, in turn, stimulates the thyroid gland to release more thyroid hormone. Similarly, when the levels of thyroid hormones are low, the TRH and TSH production is reduced, thus decreasing the release of the thyroid hormone.
What Is the Function of Thyroid Hormones?
The following are the functions of the thyroid gland:
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It controls muscle contraction.
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It regulates the basal metabolism rate (the rate at which calories are burnt), thus having an impact on weight gain or loss.
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It regulates the rate at which the dead cells are replaced.
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It controls the heart rate.
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It regulates the rate of food movement through the digestive tract.
What Are Thyroid Function Tests?
The thyroid function tests are carried out by taking blood from the arm veins. This blood is evaluated for the amount of thyroid hormones present. With the help of thyroid hormone levels, the functioning of the thyroid gland is determined.
What Conditions Are Diagnosed With Thyroid Function Tests?
In general, thyroid function tests help diagnose hyperthyroidism (increased level of thyroid hormone in the blood) and hypothyroidism (decreased level of thyroid hormone in the blood). In addition, it also helps in diagnosing other thyroid dysfunctions that are associated with hyperthyroidism and hypothyroidism.
These include:
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Thyroiditis.
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Goiter.
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Tumors of the thyroid gland.
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Thyroid nodule.
What Is the Effect of Hyperthyroidism?
Hyperthyroidism is characterized by increased metabolism, weight loss, insomnia, increased heart rate, puffiness around the eyes, anxiety, etc. When there is an increased production of thyroid hormones than the needed amount, hyperthyroidism occurs. An important cause of hyperthyroidism is Grave's disease.
What Is the Effect of Hypothyroidism?
If the thyroid hormone production is reduced than the required amount, it is called hypothyroidism. Hashimoto's disease is the most common cause of hypothyroidism. Fatigue, weight gain, skin dryness, puffiness, menstrual abnormality, etc., are the associated symptoms.
What Tests Are Included in Thyroid Function Tests?
1. T4-
The level of T4 or Thyroxine present in the blood helps diagnose hypothyroidism and hyperthyroidism. When the level of thyroxine is decreased in the blood, it denotes hypothyroidism, and an increased level indicates hyperthyroidism. For an adult, the average level of thyroxine in the blood is 5.0 – 11.0 ug/dL.
2. T3-
Decreased levels of T3 are manifested in people with hypothyroidism however, Triiodothyronine level is mainly used in the diagnosis of hyperthyroidism. Also, it helps in determining the severity of the condition. The average level of T3 in the blood is 100 - 200 ng/dL.
3. TSH-
A low level of TSH and T4 or T3 indicates a problem with the pituitary gland. As mentioned earlier, the T4 and T3 hormones level is determined by the thyroid-stimulating hormone released by the pituitary gland. A decreased amount of TSH release accompanies an increased thyroid hormone level in the blood. In the same way, a reduced level of thyroid hormone in the blood causes the pituitary gland to release more TSH, thus increasing its level in blood. Therefore, raised TSH values denote hypothyroidism, while decreased TSH values indicate hyperthyroidism. 0.40 - 4.50 mIU/mL is the average level of TSH for a healthy adult.
4. Free T3 and T4-
Some proteins usually bind with T3 and T4. As a result, the accurate measurement of the level of T3 and T4 may be difficult. In the case of free T3 or T4 measurement, the proteins are eliminated to give accurate results. Therefore, it is the most preferred test by many physicians to diagnose thyroid dysfunction. An adult's average free T3 level is 2.3 - 4.1 pg/ml, and the normal free T4 level is 0.9 - 1.7 ng/dL.
5. Calcitonin-
It is also a hormone produced by the parafollicular cells of the thyroid gland, and the normal value is any value less than 10 pg/mL. Calcitonin is used to diagnose rare thyroid disorders like medullary thyroid cancer and C-cell hyperplasia.
6. Thyroid Antibodies-
A thyroid antibody test is indicated when there is an abnormality in the level of thyroid hormones. The antibodies indicate a presence of an autoimmune thyroid disorder. Each type of antibody indicates a particular thyroid dysfunction.
There are several thyroid antibodies, which include the following:
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Thyroglobulin antibodies (Tg Ab) Hashimoto's thyroiditis.
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Thyroid peroxidase antibodies (TPOAb) Hashimoto's thyroiditis or rarely in Grave's disease
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Thyroid-stimulating hormone receptor antibodies (TSHR Ab, also known as TRAb) Grave's disease.
7. Thyroglobulin-
It is a protein synthesized by the thyroid glands used in the production of T3 and T4 hormones. Thyroglobulin helps diagnose thyroiditis and monitor thyroid cancer.
What Medications Should Be Taken With Caution Before Taking Thyroid Function Tests?
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Oral Contraceptive Pills-
When taking oral contraceptive pills, increased T3 and T4 levels are seen. This is because proteins that bind with the T3 and T4 hormones are raised due to estrogen present in the birth control pills. Therefore, it is always advisable to check free T3 or free T4 levels in such cases.
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Biotin-
An over-the-counter supplement present in many hair products impacts thyroid hormone levels. It gives a false higher value despite having an average level in blood. Therefore, it is recommended to stop Biotin at least two days before taking thyroid function tests.
What Other Tests Help in Determining Thyroid Function?
The radioiodine uptake test helps in evaluating the function of the thyroid gland. In this test, radioactive Iodine is injected and is tracked by a physician. The amount of radioactive Iodine that has been absorbed is determined, which helps ascertain the function of the thyroid gland.
Conclusion
The thyroid function test denotes the group of blood tests taken to assess the function of the thyroid gland. Although it is easy to identify if there is decreased or increased level of thyroid hormone in the blood with the help of the results, treatment should not be started on your own without the advice of a healthcare professional. Because only a professional can help you identify the cause and provide you with the best treatment.
