What is the thyroid gland?
The thyroid is a butterfly-shaped gland located in the anterior box of the neck. A normal, healthy thyroid gland is usually not visible from the outside and is difficult to palpate.
Anatomically and morphologically, the thyroid gland is divided into lobules with thyroid follicles that have an iodine-rich colloid in the center, surrounded at the periphery by thyroid cells (thyroidocytes).
The hormones produced by the thyroid, thyroxine or T4 (contains 4 iodine atoms) and triiodothyronine or T3 (contains 3 iodine atoms by removing one iodine atom) are released into the bloodstream and transported to the target organs.
What does the thyroid gland secrete?
The main hormone synthesized by the thyroid gland is T4, while T3 is partially synthesized in the thyroid cell, the rest being a product of the conversion of T4 to T3, in tissues such as liver, brain where it acts predominantly. Thyroid hormones are involved in: control of cellular metabolism, increased oxygen consumption and heat production, normal development of nerve and bone tissue, gluconeogenesis, glycogenolysis, increased intestinal glucose absorption, lipolysis.
At target organ level: it stimulates the cardio-respiratory system (heart and respiratory rate, cardiac output), at digestive level it influences intestinal motility, at bone level it increases bone turnover and bone resorption, contributes to the normal and harmonious growth and development of the body, of the brain in children and controls the proper functioning of the gonads and fertility.
How does the thyroid gland work?
The development and function of the thyroid gland is controlled by the hypothalamic-pituitary-thyroid axis and by self-regulation by iodine.
Thus, thyroid T4 secretion is under the control of another hormone, thyroid stimulating hormone (abbreviated TSH ) secreted by the pituitary gland at the base of the skull. TSH increases or decreases in inverse proportion to the level of T4 that the pituitary senses: if the pituitary signals less T4, it produces more TSH, which then tells the thyroid to increase T4 synthesis. Once the level of T4 in the blood reaches a certain level, TSH production returns to normal. Metaphorically, the relationship between TSH and T4 production is like the relationship between a thermostat and a heat source. Once the heat source is switched off, the thermostat senses the drop in temperature and switches the heat source on. When the temperature rises to the optimum level, the thermostat senses and turns off the radiator.
Although iodine is an essential substrate for thyroid hormone synthesis, its excess prevents several steps in thyroid hormone synthesis that result in inhibition of thyroid hormone release for a limited period of 10-14 days (Wolff-Chaikoff effect). This effect is useful in preoperative preparation of hyperthyroid patients with Graves’ disease or to protect against radioactive I131 clouds released during nuclear accidents.
Thyroid function tests – there are many blood tests for thyroid evaluation, however the diagnosis of each thyroid disease requires a particular set of tests recommended by the endocrinologist.
TSH – The screening test for the evaluation of thyroid function is the measurement of the TSH level by taking a blood sample. TSH is the first alarm of the system that appears when there are thyroid hormone disturbances, its level can be altered before thyroid hormones. An increased TSH level indicates an insufficiency of thyroid hormone secretion, usually primary hypothyroidism. In the opposite situation, a low TSH level indicates an excess of thyroid hormones, i.e. hyperthyroidism. If there are clinical signs of hypothyroidism, but the TSH level is not elevated, we may suspect a pituitary problem, which causes a decrease in TSH secretion and thus the thyroid is no longer adequately stimulated to produce thyroid hormones(secondary hypothyroidism).
T4 – Total circulating thyroxine measures both the protein-bound hormone circulating in the blood and the free, non protein-bound fraction. We usually measure free thyroxine (freeT4), that is the free, biologically active fraction directly responsible for its actions in the target organs. It is important to remember that fT4 is measured together with TSH when indicated. High TSH and low fT4 indicate primary hypothyroidism, while low TSH and high fT4 indicate hyperthyroidism.
T3 – T3 measurement is indicated in the diagnosis of hyperthyroidism or to assess the severity of hyperthyroidism. In hyperthyroidism, patients have low TSH, elevated fT4 and T3, or only elevated T3 with normal fT4. Testing T3 in hypothyroidism does not provide additional information, as it is the last to change. The fT3 assay is possible, but its accuracy is questionable and therefore not very helpful.
Reverse T3 is the form with no biological activity, which usually occurs in inanitia. Normally, its measurement in healthy outpatients does not identify hypothyroidism and therefore clinically it is not useful.
Thyroid anti-thyroperoxidase autoantibodies (ATPO ) and anti-thyroglobulin (anti-TG ) – TPO (thyroperoxidase) antigen is a glycoprotein enzyme attached to the thyrocyte membrane apically to the colloid. Its role is to convert inorganic iodine I- taken from food into organic iodine I2. The immune system protects us from foreign invaders such as bacteria or viruses by producing antibodies via lymphocytes. In the thyroid, T-lymphocytes have been observed that are specifically oriented to specifically stimulate B-lymphocytes producing anti-TPO autoantibodies (ATPO). Thus, in this case, we are talking about what is called thyroid autoimmunity by producing antibodies that destroy it. Another type of antibodies are anti-thyroglobulin antibodies. The presence of ATPO and/or anti-TG in a patient with hypothyroidism indicates Hashimoto’s Thyroiditis, their detection being useful only in the initial diagnosis.
TRAB- anti – TSH receptor autoantibodies – TSH receptor autoantibodies – stimulants or blockers (cause of hypothyroidism in a minority of patients, as they can cross the placenta) can also be dosed. TRAb are present in about 90% of patients with Graves’ disease (autoimmune hyperthyroidism) and almost never in a patient with another form of autoimmune chronic thyroid disease. They are assayed in several situations: in a pregnant woman to determine the risk of fetal or neonatal hyperthyroidism in the infant (in case of placental crossing of TRAb), positive diagnosis of Graves’ disease with normal thyroid function and ophthalmopathy (eye damage), to assess the response to treatment of hyperthyroidism and to predict the risk of relapse in a patient with Graves’ disease.
Thyroglobulin – Thyroglobulin is a protein, a major component of follicular colloid. The only usefulness for dosage is in postoperative follow-up of patients who have undergone surgery for thyroid cancer, according to which we adapt the therapeutic management.
Medications that interfere with thyroid function testing:
The estrogens present in oral contraceptives cause an increased level of TT4 and T3, by increasing the hepatic production of binding proteins involved in their transportation, In this case, TSH and fT4 testing is indicated, which is usually within normal parameters.
Biotin is a supplement that may cause interference with the test kit and lead to inconclusive results. It is recommended to stop taking biotin two days before collection to avoid this effect.
Referinte:
h#ps://www.thyroid.org/
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