Thyroid gland is a small butterfly shaped gland present at the base of the neck. It is responsible for the production of hormones such as thyroxine (T4), triiodothyronine(T3) and calcitonin that are important for regulating the body's metabolism.
What do thyroid hormones do?
The thyroid hormones affect each and every cell of the body and regulates:
• How we breathe
• How fast or slow our heart beats
• How our body digests food for energy
• How we maintain our body weight
• How happy or sad we feel
How does thyroid gland work?
The need of thyroid hormones in the body varies based on our physiological state. The pituitary gland in our brain, secretes the thyroid stimulating hormone to convey to the thyroid gland about how much thyroid hormones is needed by the body. Therefore, the thyroid gland using the iodine from the food, produces the exact amount of thyroid hormones to help regulate body functions.
What happens when the thyroid gland does not function well?
When your thyroid is not secreting enough hormones, it means that you have an underactive thyroid. This can lead to symptoms of hypothyroidism such as:
• tiredness
• sensitivity to cold
• weight gain
• constipation
• depression
• muscle aches
• weakness
Overactive thyroid means that your thyroid is secreting excess hormones causing hyperthyroidism. The symptoms of overactive thyroid include:
• anxiety
• difficulty in sleeping
• tiredness
• sensitivity to heat
• diarrhea
• persistent thirst
• itchiness
Thyroid function tests (TFTs)
Thyroid function Test (TFTs) or thyroid profile test are simple blood tests that are used to measure the level of thyroid hormones and help in assessing the functioning of the thyroid gland.
Thyroid Stimulating Hormone (TSH) test: Thyroid stimulating hormone is secreted in the blood by the pituitary gland and it stimulates the thyroid gland to produce T3 and T4 hormones. Hypothyroidism (thyroid hormone deficiency) is usually associated with high TSH levels, while hyperthyroidism (thyroid hormone excess) is associated with low TSH levels. The TSH test is the most common test used to identify thyroid diseases. It is recommended as an initial test before starting thyroid treatment.
The normal range of TSH in adults is 0.4 - 4.5 mIu/L
When TSH level is high, it means lesser hormones are being produced and TSH is stimulating the thyroid gland to produce more T4 hormones. This condition is called hypothyroidism. TSH levels above 4.5 mIU/L might be indicative of hypothyroidism.
When TSH level is low, it is indicative of an overactive thyroid with T3 and T4 levels being high, and this condition is called hyperthyroidism. Sometimes, a low TSH level could be because of a pituitary gland abnormality that prevents it from making adequate TSH. TSH levels below 3.5 mIU/L may indicate hyperthyroidism.
Thyroxine (T4) test: T4 or thyroxine is a hormone produced by the thyroid gland and there are two forms of T4 present in the body, bound T4 and free T4. Bound T4 is attached to proteins and cannot be used by the body. The metabolic functions in the body are performed by free T4. A total T4 test is used to measure both free T4 and bound T4 whereas a free T4 test is used to measure only the amount of free T4 in the blood sample. High TSH levels with low free T4 and total T4 are commonly seen in hypothyroidism.
In adults, the normal level of total T4 ranges from 5-11(mcg/dl).
Free Thyroxine (Free T4) test: It is a method of measuring T4 that eliminates the effect of proteins that naturally bind to T4 and interfere with accurate measurements.
In adults, the normal level of free T4 ranges from 0.9-1.7 (ng/dl).
Triiodothyronine (T3) test: This test helps to diagnose hyperthyroidism or to show the severity of hyperthyroidism when T3 levels are elevated. T3 is the active form of the hormone needed by the body therefore T4 is converted to T3 to perform various functions. There are two forms of T3 present in the body, bound T3 and free T3. Bound T3 is attached to transport proteins whereas free T3 is free is utilized by the cells.
The normal level of total T3 in adults ranges from 100-200 ng/dl.
Free Triiodothyronine (Free T3) test : It is a method of measuring T3 that eliminates the effect of proteins that naturally bind to T3 and interfere with accurate measurements.
The normal level of free T3 ranges in adults from 2.3-4.1 pg/ml.
Thyroid antibody profile
Thyroid antibody tests are used for diagnosing hypothyroidism or hyperthyroidism caused due to an autoimmune thyroid disease such as Hashimoto's thyroiditis and Grave's disease.
Thyroid antibody testing looks for different types of thyroid antibodies including:
Thyroid peroxidase antibodies (TPOAb): The thyroid peroxidase enzyme is essential for producing thyroid hormones, T4 and T3. TPOAb interferes with the action of this enzyme. Most patients with Hashimoto’s thyroiditis have high levels of TPOAb.
The normal level of TPO antibodies is <9 IU/mL. If the TPO results are negative, it means that there is no autoimmune disorder. If the test results are positive, it means you might have Hashimoto’s thyroiditis that cause hypothyroidism.
Thyroglobulin antibodies (TgAb): Thyroglobulin is essential for the formation of thyroid hormones. Thyroglobulin antibodies may be present when the thyroid gland has been damaged resulting in hypothyroidism.
The normal values of thyroglobulin antibodies values is <4 IU/mL. Thyroglobulin antibodies levels higher than 4 IU/mL may be caused due to Hashimoto’s thyroiditis.
TSH receptor antibodies:
TSH works by binding to structures called TSH receptors on the thyroid gland. These antibodies stimulate the thyroid gland to produce more thyroid hormones. This results in hyperthyroidism condition. In Graves’ disease, thyroid-stimulating immunoglobulin (TSI) antibody binds to the TSH receptor and mimics the function of TSH.
Normal values of TSH receptor antibodies are < 1.75 IU/L. Higher levels of TSH receptor antibodies may be caused due to Grave’s disease (autoimmune hyperthyroidism).
Thyroid profile in pregnancy
Thyroid disease is common in pregnancy as the thyroid gland undergoes several changes during pregnancy. These changes can lead to elevated thyroxine (T4)-binding globulin, total T4, total triiodothyronine (T3), and serum thyroglobulin (Tg) levels, as well as low levels of TSH and serum free T4. Concentrations of these hormones varies between trimesters therefore the reference intervals for thyroid tests must be adjusted during pregnancy to detect thyroid disease.
Blood tests for TSH, thyroid hormones and autoantibody testing to assess thyroid autoimmunity are recommended for pregnant women.
TSH test, T4, T3 and thyroid antibodies test are used to find the cause of thyroid disease, which is the is the first step in the treatment. A comprehensive test panel helps in monitoring the response to treatment for diseases like hyperthyroidism and also to evaluate patients receiving hormone therapy which replaces thyroid hormones that are depleted. Thus, getting your thyroid profile done is recommended to detect or diagnose thyroid diseases.
