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Thyroid hormones are thyroxine, triiodothyronine and calcitonin produced by this gland. The first two of these are especially important because without them a person is not able to function properly. The disease occurs both with an excess of thyroid hormones and as a result of their deficiency. What are the actions of thyroid hormones and what conditions can lead to disturbances in their amount in the body?

Thyroid hormonesare essential to life. The thyroid gland is a small organ - its weight usually reaches 60 grams. Some people are not even aware of having it, and in fact this gland and the hormones it produces are simply necessary for life. The main products of the thyroid gland are thyroxine (T4) and triiodothyronine (T3), both of which have essentially similar effects, and the gland produces a third substance, calcitonin.

Thyroid hormones: thyroxine and triiodothyronine

The most important products of the thyroid gland are thyroxine and triiodothyronine. The initial substrate for the production of these hormones is the amino acid tyrosine, hormones in their final form contain iodine atoms in their molecules. Within the thyroid gland, these hormones are produced by the transformation of the larger molecule, thyroglobulin. Thyroglobulin is stored inside the thyroid follicles, it is located within the so-called colloid, and - after the thyroid cells are stimulated by TSH - the thyroid hormones are released from it ready for actionthyroid hormones .

The thyroid gland releases mainly thyroxine, triiodothyronine is released by the gland in trace amounts. However, it is not T4, and T3 is definitely the more active hormone - its activity is 3 to 5 times greater than that of T4. The final amount of T3 in the blood, however, does not only depend on the amount released from the thyroid gland. In many peripheral tissues (including the liver, heart, intestine, pituitary gland and skeletal muscles) there is an enzyme deiodinase, thanks to which T4 is converted to much more active T3. The activity of thyroid hormones is determined not only by their amount in the blood, but also the degree of binding of these molecules to the proteins that transport them.

Only those hormones that are in free, unbound form are active. Free T4 concentration inIn the blood, it is usually about 0.03% of the total amount of thyroid hormones in the blood, and the concentration of free T3 is around 0.3%. The remaining amount of thyroid hormones is associated with albumin (they transport up to 20% of all thyroid hormones in the blood) and withthyroid hormone binding proteins(which together transport up to 85% of the total amount produced by the thyroid gland) hormones).

Thyroid hormones: regulation of T3 and T4 secretion

The secretion of thyroid hormones is controlled by three organs, which are the hypothalamus, pituitary gland and the thyroid gland itself. The first of these organs, the hypothalamus, produces the hormone thyreoliberin (TRH). This substance affects the pituitary gland, which - when stimulated by TRH - releases thyrotropin (TSH). TSH, in turn, affects the thyroid gland itself, stimulating it to secrete T3 and T4. The hypothalamic-pituitary-thyroid axis is a self-regulating system. Above is given the sequence of the phenomena that lead to the stimulation of the release of thyroid hormones. Regulation also includes the effects of T3 and T4 on the pituitary and hypothalamus - when blood levels of T3 and T4 increase, the release of TSH and TRH is reduced. The hypothalamus and pituitary gland increase their hormone production when the amount of T3 and T4 decreases again.

Thyroid hormones: T3 and T4 action in the body

Thethyroid hormonesare, broadly speaking, necessary for the proper functioning of the human body. The activities of T3 and T4 include a number of processes, such as:

  • control of metabolic processes (e.g. in the liver, these hormones stimulate the processes of gluconeogenesis and lipogenesis, and also stimulate glycogenolysis)
  • stimulation of bone growth in length by stimulating the mineralization processes
  • stimulation of the maturation of the structures of the nervous system
  • effects on the heart, including increasing its activity as well as increasing cardiac output
  • breathing stimulation
  • potentiating the action of catecholamines (thanks to thyroid hormones, tissues become more sensitive to the effects of these substances),
  • enhancing metabolic processes throughout the body
  • influencing the thickness of the uterine mucosa in women

The range of thyroid hormone activity is therefore extremely wide. The effects of these hormones appear after these compounds enter the body's cells, where nuclear receptors for thyroid hormones exist. When these hormones bind with their specific receptors, the expression of genes changesto function, the body needs a quantity of thyroid hormones adjusted to its needs. Negative effects are exerted by both the deficiency of T3 and T4, as well as their excess in the body.

Thyroid hormones: symptoms and causes of T3 and T4 deficiency

In a situation where a patient has a deficiency of thyroid hormones, he may have :

  • constant feeling of fatigue
  • pale and dry skin
  • slow heart rate
  • tendency to swelling
  • constipation
  • cold intolerance
  • concentration disorders
  • change in tone of voice (related to thickening of the vocal folds)
  • unjustified weight gain
  • hair breakage
  • memory problems
  • menstrual disorders

There are three main groups of causes of hypothyroidism. The first to be classified is primary hypothyroidism, which is caused by a pathology within the thyroid gland itself. It may occur, for example, as a result of autoimmune processes (such as in the case of Hashimoto's disease or postpartum thyroiditis), but also due to iodine deficiency in the diet. Hypothyroidism can be congenital, and the condition can also be a consequence of various medications (e.g. amiodarone).

Primary hypothyroidism can also be caused by radioiodine treatment or thyroidectomy. The remaining groups of disorders are secondary hypothyroidism (caused by insufficient secretion of TSH by the pituitary gland) and tertiary hypothyroidism (caused by inadequate release of TRH by the hypothalamus).

Thyroid hormones: symptoms and causes of excess T3 and T4

The opposite of hypothyroidism is the state of hyperthyroidism. The symptoms appearing in its course are somewhat opposite to those listed above and may be:

  • excessive sweating
  • increased heart rate
  • heat intolerance
  • diarrhea (here it should be emphasized, however, that in an overactive thyroid gland it can also lead to constipation)
  • shortness of breath
  • unexplained weight loss
  • insomnia
  • weakness
  • irritability
  • muscle tremors
  • menstrual disorders
  • skin warmth and moisture increase

Hyperthyroidism can be caused by autoimmune processes (such as Graves' disease), as well as the presence of nodules secreting thyroid hormones in the parenchyma of the thyroid gland. The disorder may also appear inthe course of postpartum thyroiditis, as well as in the case of excessive secretion of TSH by the pituitary gland. Occasionally, hyperthyroidism is caused by excess thyroid hormone ingestion by patients suffering from hypothyroidism.

Thyroid hormones: calcitonin

When discussing thyroid hormones, usually much less attention is paid to calcitonin than to thyroxine or triiodothyronine, but this does not mean that the last of these substances is not important for the functioning of the human body. Calcitonin is produced mainly by the thyroid gland and takes place in the follicular cells (also known as C cells). However, this polypeptide is also synthesized by the parathyroid glands and thymus, although in much smaller amounts. The biological function of calcitonin is to control the body's calcium metabolism. It is the amount of this substance in the blood that determines the release of calcitonin - in the case of this hormone, the hypothalamus and the pituitary are not involved in the control of its secretion. Calcitonin is released when the amount of calcium in the blood increases. The action of this hormone is based on the inhibition of the activity of osteoclasts (these are the cells that release calcium from the bones), as well as the inhibition of calcium resorption within the renal tubules (thus increasing the loss of calcium in the urine). By exerting the described effects, calcitonin is antagonistic to the parathyroid hormone secreted by the parathyroid glands.

Thyroid hormones: diagnosis

Determining the level of TSH is of primary importance in the diagnosis of disorders related to the thyroid function. Already on the basis of the TSH test itself, one can conclude about the presence of potential diseases - the TSH standard is usually 0.2-4.0 µU per milliliter of blood (this standard, however, varies depending on, among others, the patient's age, it is also different for pregnant patients). The low level of TSH most generally suggests the existence of hyperthyroidism, while high values ​​of this hormone indicate the presence of hypothyroidism in the patient. Other indications for the diagnosis of thyroid diseases are:

  • amount of T3 and T4 (especially free ones) in the blood
  • anti-thyroid antibodies (e.g. anti-thyroid-peroxidase, anti-TSH-receptor or anti-thyroglobulin)

More specialized tests, such as the TRH test (performed in patients with abnormal TSH levels, to differentiate whetherdeviations in TSH are related to the improper function of the pituitary gland or pathology within the thyroid gland). As for calcitonin, its determination - despite the function of this hormone - is not carried out mainly when there is a suspicion of disturbances in the economy. calcium. Calcitonin measurement is useful primarily in the diagnosis and monitoring of patients with medullary thyroid cancer - calcitonin is a marker of this cancer.

Thyroid Hormones: Applications to Treat Various Conditions

Thyroid hormonesin medicine are used primarily in patients with hypothyroidism. The most important in this case are levothyroxine preparations, but sometimes mixtures with triiodothyronine derivatives are also used. Calcitonin administration may, in turn, treat osteoporosis, hypercalcemia and Paget's disease. Occasionally, calcitonin is used in patients with bone metastases because it helps to relieve patients' pain.

Read also:

  • Hormones
  • Thyroid
About the authorBow. Tomasz NęckiA graduate of medicine at the Medical University of Poznań. An admirer of the Polish sea (most willingly strolling along its shores with headphones in his ears), cats and books. In working with patients, he focuses on always listening to them and spending as much time as they need.

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