The thyroid gland contains follicular cells and C cells. The follicular cells in the thyroid gland are also called thyroid. The main trigger of thyroid hormones is thyrothypine releasing hormone (TRH), which is secreted from the hypothalamus and sends instructions to the pituitary. TRH sends a warning to the pituitary gland, causing the pituitary to secrete thyroid synthesizing hormone (TSH) (thyrothypine). TSH secreted from the pituitary gland stimulates the thyroid follicular cells and ensures the secretion of T4 (tychosine) and T3 (triiodothyronine) hormones from these cells. Most T4 and a small amount of T3 are secreted. T4 (thyroxine) is the main thyroid hormone composed of tyrosine amino acids and iodine compounds. Therefore, iodine is one of the main raw materials of thyroid hormone production.
Thyroid follicular cells also have the ability to synthesize thyroglobulin, concentrate iodide from the bloodstream to the thyroid, and carry TSH receptors.
C cells in the thyroid gland are located between the main follicular structure and in the follicle walls. These cells secrete the hormone calcitonin.
The secretion of thyroid hormones (T4 and T3) depends on the regularly functioning follicular cells, the presence of iodide in the medium, thyroglobulin adequacy, the adequacy of the thyroid peroxidase enzyme in the thyroglobulin, the stimulation of the thyroid gland by the TSH hormone and the amount of storage thyroid hormones on the thyroglobulin colloidal tissue. Thyroglobulin colloidal tissue is synthesized by thyroid follicular cells. This tissue contains the thyroid peroxidase enzyme, which ensures the secretion of thyroid hormones. TSH stimulus activates the thyroid peroxidase enzyme in thyroglobulin and provides hormone secretion.
T3 is the active form of thyroid hormones. Although it is secreted in small amounts from the thyroid gland, it is mainly activated by the conversion of the T4 hormone to T3. T3 activation occurs with the separation of iodide from the T4 hormone. The released iodine is preserved and prepared for reuse.
The increase in T3 and T4 hormones stimulates the pituitary glands and suppresses TSH secretion, which in turn suppresses TRH secretion from the hypothalamus and the cycle continues in this way.
Monitoring of thyroid function usually begins with an analysis of the TSH hormone level. If TSH blood values are outside the reference range of 0.4 to 4.5 milliunits/ml and TSH elevation is tested, T4 hormone blood level is measured. If the TSH value is below the normal range, both T4 and T3 blood levels are measured.
Since TSH hormone levels are stimulating for the secretion of T4 and T3 hormones, when the blood T4 and T3 hormones are low, more TSH is secreted and tries to increase this secretion. If T4 and T3 levels are not enough, TSH level will rise above normal. This happens in the case of hypothyroidism.
In addition to low T4 levels in Hashimoto’s disease, high creatine kinase levels, high cholesterol levels and high thyroid antibodies may accompany.
Thyroid hormones can be seen at different levels in many diseases of the thyroid glands. In the diagnosis and monitoring of these diseases, thyroid hormones should be measured from time to time. These evaluations may require not only the measurement of thyroid hormones, but also the use of many different tests and imaging methods.
Abnormalities in TRH and TSH chains can also cause abnormalities of normally functioning thyroids. For example, anomalies (tumors, infections, etc.) that cause excessive TRH (Tyrothyropine releazing hormone) secretion from the hypothalamus may cause TSH secretion indirectly from the pituitary gland. This excessive TSH level can cause the thyroid gland to secrete excessive thyroid hormone. Conversely, conditions that suppress TRH secretion from the hypothalamus or the causes that reduce TSH secretion from the pituitary gland may cause lower levels of thyroid hormone secretion from the thyroid gland.
Functions of thyroid hormones
Some of the main functions of thyroid hormones are:
- Thyroid hormones contribute to fetal brain development and skeletal maturation from the 11th week.
- Thyroid hormones increase oxygen consumption and heat production in all cells except the brain, spleen and testis. It accelerates the basal metabolism in the whole body.
- Thyroid hormones help in the overall growth, development and differentiation of all body cells.
- Thyroid hormones release energy by increasing protein turnover in skeletal muscle and regulate basal metabolic rate.
- In case of low blood oxygen level, they increase the oxygen level of the blood by stimulating the respiratory muscles.
- Thyroid hormones facilitate the separation of oxygen from hemoglobin in erythrocytes, making it easier for tissues to reach oxygen.
- They play an important role in calcium metabolism.
- They help the overall development and function of the central nervous system in children. Mental retardation can develop in thyroid hormone deficiency at young ages.
- By promoting somatic and psychic growth, they affect the body’s mood and behavior.
- T3 facilitates the relaxation of the heart and accelerates its contraction. It also increases the heart rate by accelerating the sinoatrial nodes of the heart.
- Thyroid hormones stimulate bone turnover. They help build up calcium and phosphate in the bone and strengthen bones.
- They lower the level of calcium in the blood.
- They regulate carbohydrate, fat and protein metabolism.
- They also aid in the metabolism of vitamins.
- They affect the hormonal balance by affecting the production and metabolic processes of some hormones. They accelerate growth by increasing the effect of growth hormones in particular.
- They help lower cholesterol and triglycerides.
- They maintain electrolyte balance.
- They support erythrocyte production.
- They promote cell development by accelerating mitochondrial metabolism in cells.
- Thyroid hormones stimulate intestinal motility by regulating intestinal motility. They also cause increased defecation.
- They also increase the sensitivity of beta-adrenergic receptors to catecholamines.
Thyroid hormones are transported in the blood bound to plasma proteins. The main proteins that carry thyroid hormones in the blood are:
- Thyroxine binding globulin (TBG) (70%)
- Transthyretin (prealbumin) (10%)
- Albumin (15%)