The meaning of test indicators to assess thyroid function

The article was professionally consulted with General Internal Medicine Doctor - Department of Examination & Internal Medicine - Vinmec Nha Trang International General Hospital.

Tests to evaluate thyroid function including TSH, total T4, FT3, TSI and others play an important role in the diagnosis and treatment of thyroid diseases.

The thyroid is a small butterfly-shaped gland located in the front of the neck. The main function of the thyroid gland is to produce hormones that regulate the functioning of cells and organ tissues in the body. Thyroid hormones affect the body's metabolic rate and the levels of certain minerals in the blood. The thyroid gland produces 3 hormones and releases them into the bloodstream. Two of them, thyroxine (T4) and triiodothyronine (T3), are responsible for speeding up the body's metabolism. The other hormone is responsible for controlling calcium levels in the blood.
To produce T3 and T4, the thyroid gland needs iodine. Called T4 for short because in its molecule it has 4 iodine atoms and similarly, T3 contains 3 iodine atoms. In the cells and tissues of the body, T4 is reduced by 1 iodine to convert it to T3. T3 has 5 times more biological activity than T4, affecting the activities of cells and body tissues.
Thyroid activity is controlled by hormones produced by the hypothalamus and pituitary gland of the brain. The hypothalamus receives signals about the state of functions in the body. When the hypothalamus senses low T3 and T4 levels or the body's metabolic rate is low, it releases thyrotropin-releasing hormone (TRH). TRH travels to the pituitary gland via the vascular system, stimulating the pituitary gland to secrete thyroid-stimulating hormone (TSH).
TSH is produced from the pituitary gland into the bloodstream, going to the thyroid gland. In the thyroid gland, TSH stimulates thyroid cells to produce more T3 and T4. Then, T3 and T4 will be released into the blood, which increases the metabolic activity of cells in the body.

Thyroid tests include:
Thyroid-stimulating hormone (TSH): The pituitary hormone that sends signals to the thyroid gland. If the pituitary gland detects that there is less thyroid hormone in the blood, it will produce more TSH to prompt the thyroid gland to produce more thyroid hormone. In case the pituitary gland detects that there is too much thyroid hormone in the blood, it will reduce the production of TSH to reduce the production of thyroid hormone; Thyroxine (T4): The total T4 test helps measure the total amount of thyroxine in the blood, measuring thyroid function. However, the measurement of total T4 is strongly influenced by the amount of protein in the blood because the protein can bind T4 to red blood cells, turning T4 into the active form. In contrast, free T4 (Free T4 - FT4) is unaffected by the amount of protein in the blood, which is considered the active form of thyroxine. If a new problem is suspected, a T4 test will be done along with a TSH test; Triiodothyronine (T3): An active form of thyroid hormone, made from T4. The total T3 test measures the amount of circulating Triiodothyronine in the blood (both protein bound and nonprotein bound T3). However, only the protein-bound T3 is capable of transporting oxygen and energy to cells. The free T3 test (Free T3 - FT3) only measures the amount of protein bound T3 (T3 in the active form); Thyroid peroxidase antibody (TPOAb): An antibody produced by the body that inadvertently attacks and destroys healthy thyroid tissue. The presence of this antibody indicates that the patient has an autoimmune thyroid disease such as Hashimoto's or Grave's thyroiditis. And TSI (Thyroid Stimulating Immune Globulin) is an antibody that stimulates the thyroid gland to increase its activity and overproduce thyroid hormone into the bloodstream; Thyroglobulin (Tg): A protein produced by the thyroid gland. The presence of Tg in the blood is an indication that thyroid tissue remains after thyroidectomy or radiation therapy. This test is valuable in patients with thyroid cancer to detect whether the cancer cells are still producing Tg or not compared to before cancer treatment; determine the results of cancer treatment and detect the possibility of cancer recurrence after treatment; Thyroglobulin Antibody (TgAb): An antibody produced by the body in response to the presence of Thyroglobulin. Excessive thyroglobulin production is abnormal. Therefore, TgAb production is considered the body's choice of defense against the progression of thyroid disease. The thyroid function test indicators in normal healthy people are:
TSH index: 0.4-4.0 mU/L; T4 index: 60-140 nmol/L; FT4: 10-26 pmol/L; T3 index: 1.1-2.7 nmol/L; FT3 index: 3.5-7.8 pmol/L.

High TSH and low FT4 indicate primary hypothyroidism due to thyroid disease such as Hashimoto's thyroiditis; Low TSH and low FT4 indicate hypothyroidism secondary to pituitary disease or a response to non-thyroid disease; Low TSH and elevated FT4 indicate hyperthyroidism, such as Graves' disease (Basedow); In case the TSH index increases slightly but the FT4 index is still within the normal reference range, it is a warning sign of asymptomatic hypothyroidism; If initial thyroid test results suggest thyroid dysfunction or autoimmune thyroid disease is suspected, your doctor may order one or more additional thyroid antibody tests such as: TPOAb, TgAb and TRAb. Thyroid hormone levels can be influenced by a number of factors such as: different time of day, different labs, pregnancy, age, medications used to treat thyroid disorders, serious illness, effects after consuming certain foods,...

If you have symptoms such as unexplained rapid weight loss, sweating, fast heartbeat, fatigue, eye irritation, frequent bowel movements or diarrhea, etc. or symptoms related to hyperthyroidism, It is best for patients to go to a large hospital to be diagnosed and tested for thyroid function.
Currently, Vinmec International General Hospital is the address that provides endocrinology packages, tests to evaluate thyroid function and active and effective disease treatment for all patients. The hospital's Endocrinology Department is a gathering place of experienced doctors, solid expertise and always devoted to the patient's health. At the same time, the hospital is also equipped with a system of modern medical equipment, comfortable facilities, the best support for the diagnosis and treatment of diseases.