The thyroid hormones I'm there triiodothyronine (T3) and the thyroxine (T4). The thyroid hormones have an important function for the body: when our body is subjected to strong physical or psychological stress, or when the levels of thyroid hormones decrease in the bloodstream, a part of the brain, the hypothalamus, is stimulated to produce and release the TRH, i.e. thethyrotropin-releasing hormone. TRH in turn acts on the anterior lobe of the pituitary gland, the adenohypophysis, and stimulates the production of TSH, also called thyrotropin, thyrotropic hormone or thyroid stimulating hormone. TSH stimulates the thyroid to produce and release into the circulation thyroid hormones.
T3 or triiodothyronine: 100–200 ng / dL (1.54-3.08 nmol / L according to the SI unit of measurement, the International System Units)
fT3 or free triiodothyronine: 2.6–4.8 pg / ml (4.0–7.4 pmol / L according to the SI unit of measurement, the International System Units)
T4 or thyroxine: 4.5–11.2 μg / dL (58–144 nmol / L according to the SI unit of measurement, the International System Units)
fT4 or free thyroxine: 0.8–2.7 ng / dL (10–35 pmol / L according to the SI unit of measurement, the International System Units)
The thyroid hormones are produced by thyroid, a butterfly-shaped gland located at the front of the neck. Thyroid hormones are synthesized from thyroglobulin, in turn dependent on the presence of iodine (a deficiency of iodine will therefore cause a deficiency of thyroid hormones) and dal tyrosine, an amino acid precursor to other substances such as catecholamines (dopamine, adrenaline, noradrenaline) and melanin. The main regulator of the thyroid is the TSH (thyroid stimulating hormone o Thyroid-stimulating hormone), also called thyrotropin or, a hormone produced in the pituitary, which in turn is stimulated by a hormone produced in the thalamus, the TRH (thyrotropin-releasing hormone). Low levels of thyroid hormones stimulate greater production of TRH and increase TRH receptors in the pituitary cells: consequently, the pituitary gland produces greater quantities of TSH, which acts directly on the thyroid cells and on the various thyroid enzymes, resulting in greater hormone production. The reverse happens when there is too much thyroid hormone in the bloodstream.
Thyroid hormones are represented by the thyroxine (T4) for over 90%. T4 is then converted at the peripheral intracellular level by the 5-deiodinase a enzyme triiodothyronine (T3), a hormone at least 5 times more active than thyroxine. There is one for both thyroid hormones free fraction o "free" (indicated with an f before the hormone name: fT3 and fT4), and a portion bound to plasma proteins, such as thyroxine-binding globulin, TBG), albumin and the transthyretin. For example, approximately 99.7% of T3 is bound to plasma proteins, and only 0.3% is found in the form of fT3. The proteins that bind thyroid hormones are never completely saturated: TBG, for example, in physiological conditions is saturated for less than a third of its total availability. The ratio between the fraction of protein-bound thyroid hormones and the free fraction depends on the body's energy needs and on the activity of the thyroid.
In case of hyperthyroidism there will be an excessive amount of thyroid hormones in circulation: to remedy this disproportion the fraction linked to TBG and other plasma proteins will increase, trying to keep a level of thyroxine and triiodothyronine as close as possible to normal values. In hyperthyroidism, due to the hypothalamic-pituitary self-regulation mechanism which is suppressed by high thyroid hormone values, low TSH values are more common.
In some cases you may be faced with a "false hyperthyroidism”In this situation they show themselves high total values of thyroxine or triiodothyronine, but without the patient showing the typical signs and symptoms of hyperthyroidism; this is the case, for example, of pregnant women or people on therapy with drugs containing estrogen: in these situations, the high level of estrogen can increase the synthesis of TBG and consequently an increase in the amount of protein-bound thyroid hormones . The free fraction of thyroid hormones will decrease accordingly, and the thyroid, stimulated by TSH, will try to compensate by producing a greater quantity of thyroid hormones: in conclusion, in the blood tests we will have normal values of fT3 and fT4 and high values of total T3 and T4 .
In other cases, a "false hypothyroidism": This occurs during cortisone-based therapies, or in the presence of liver diseases: in both cases there may be a decreased production of TBG and other proteins binding thyroid hormones. The fraction bound to proteins will decrease, and consequently the fraction of free thyroxine and triiodothyronine will increase: this will lead to a decrease in the "productive push" exerted by the pituitary gland on the thyroid through TSH, and low total values may be found in blood tests of T3 and T4, with normal values of fT3 and fT4.
Thyroid hormones are essential for many primary functions of our body, such as the production of energy by controlling the enzymes responsible for energy metabolism, and the processes of growth and development of the organism.
The action of thyroid hormones is manifested for example at the cardiac level: fT3 in particular exhibits a positive inotropic and chronotropic effect, i.e. causes an increase in range and heart rate, allowing proper blood supply to the organs. Too low levels of fT3 and fT4 can contribute to the development of heart failure; conversely, too high levels of triiodothyronine and thyroxine will cause tachycardia and exaggerated heart pump activity.
To counterbalance the increased oxygen consumption linked to increased cardiac activity, fT3 and fT4 they also increase the availability of oxygen facilitating its release from the hemoglobin molecule, e favoring hematopoiesis, i.e. the production of red blood cells and hemoglobin, through the increased production of EPO (erythropoietin).
Thyroid hormones are essential in learning and memory processes, in keeping us reactive to stimuli external and with adequate state of vigilance.
Under the influence of thyroid hormones they come mobilize fatty substances (lipids and fatty acids) of our body and sugars (carbohydrates), contributing to thermogenesis, which is the production of heat in our body. People with thyroxine and triiodothyronine deficiency have difficulty adapting to the cold, as their bodies cannot "burn" enough substances to generate sufficient heat to maintain an adequate body temperature.
Thyroxine and triiodothyronine also promote the dermal, bone and brain development of the fetus, contributing to the ossification and growth of nerve fibers, myelination of axons and branching of dendrites and synapses, growth of the cerebral cortex and cerebellum. This is the reason why infants who have suffered from a reduced intake of fT3 and fT4 during the neonatal period may have severe mental retardation (infantile cretinism).
Triiodothyronine participates in the bone remodeling also in adults: in case of hyperthyroidism, the risk of osteoporosis increases.
The finding of high values of free and total thyroid hormones allows us to diagnose the presence of hyperthyroidism. The main symptoms in the presence of high fT3 and T3 and high fT4 and T4 can be:
- increased heart rate
- weight loss
- difficulty falling asleep and / or awakening at night
- widespread tremors
- vision disturbances (photophobia)
- tiredness alternating with a feeling of excitement
The finding of low values of free and total thyroid hormones allows us to diagnose the presence of hypothyroidism. The main symptoms in the presence of T4 / fT4 or low total and free thyroxine and low T3 / fT3 or total and free triiodothyronine may be:
- weight gain
- dryness and brittleness of the skin
- fragility of hair and nails
- hair loss
- menstrual cycle irregularities
- In severe cases, it can also occur:
- heart failure,
- loss of alertness up to coma
- in children: delayed brain and bone development
The examination is carried out starting from a blood sample obtained with a venous sampling. It is not necessary to fast before the exam. Some drugs could interfere with the thyroid hormone values: for this reason it is necessary to inform those who will evaluate the results of all the therapy in place at the time of the tests.
[sociallocker id = ”403 ″] Thyroid hormones are measured to evaluate thyroid function and identify any alterations both in the sense of thyroid hyperfunction (hyperthyroidism) and in case of hypoactivity (hypothyroidism); measuring T3 and T4 is also essential, together with the TSH measurement, to check the efficacy of therapeutic treatment in case of hypothyroidism (monitoring of thyroid hormone replacement therapy, whose best known trade name is Eutirox) or hyperthyroidism (monitoring thyroid suppressive therapy).
The diagnosis is made through the test conducted on a blood sample obtained with a venous sample, which measures the amount of thyroid hormones present in the circulation. Usually with the same sample the doctor also recommends the TSH dosage, which is essential in order to establish the possible cause of high levels of fT3 and fT4.
If autoimmune thyroid disorders are suspected, thyroid hormones and TSH are associated with the search for anti-thyroid antibodies, namely the antibody anti thyroglobulin (ab anti-TG), which is a precursor molecule from which thyroid hormones are produced, theanti-TSH receptor antibody (ab anti-TSH), and the antibody thyroid antioxidase or thyroid peroxidase (ab anti-TPO), directed against thyroperoxidase, which is a protein with enzymatic function that regulates the production of thyroid hormones.
To get a clearer picture, these analyzes are often combined with other blood tests, such as blood count, kidney function (urea and creatinine), mineral salts in the blood (sodium, potassium, less frequently calcium and magnesium), and inflammation parameters, i.e. the C reactive protein (PCR), and the erythrocyte sedimentation rate (VES).
From the radiological point of view, if there is an evident morphological alteration (for example a nodule palpable or visible, or a thyroid goiter), or if you want to investigate any alterations in blood tests, the first level exam and aultrasound of the thyroid gland. Only secondarily, if necessary, will it be possible to make one thyroid scintigraphy and possibly a fine needle aspiration. [/ sociallocker]