The TSH is a hormone produced by the pituitary gland, it can also be called thyrotropin, thyrotropic hormone or thyroid stimulating hormone. The analysis of TSH values allows us to understand how the gland is working thyroid. A Low TSH it is often linked to increased thyroid hormone levels with symptoms of hyperthyroidism.
The TSH is the main regulator of two thyroid hormones: the T4 (thyroxine) and T3 (triiodothyronine). Thyroid hormones are essential substances for the proper functioning of the body, and are synthesized from a precursor called thyroglobulin.
In the blood tests the part of the thyroid hormones not bound to proteins, ie the "free" part of the thyroid hormones. These hormones are often referred to as fT3 and fT4, where "f" means "free".
Given that each laboratory has its own reference ranges, we speak of Low TSH when its concentration measured by a peripheral blood sample turns out to be less than 0.5 mIU / L.
Thyrotropin (TSH) is measured to evaluate thyroid function and identify any alterations both in the sense of thyroid hyperactivity (hyperthyroidism), and in case of hypofunction (hypothyroidism); measuring TSH also serves to monitor the effectiveness of therapeutic treatment in case of hypothyroidism (monitoring of thyroid hormone replacement therapy) or hyperthyroidism (monitoring of thyroid suppressive therapy).
Sometimes in the doctor's prescriptions or in blood tests we find the wording TSH reflex or TSH reflex: this is not a different type of TSH, but the name of a protocol that aims to optimize medical expenses by first performing only the TSH dosage and then, if this is altered, continues with the dosage, on the same tube of blood, of the thyroid hormones. Hence TSH reflex and TSH both indicate thyrotropin hormone.
What does it mean have a low TSH? What does it mean when do we find low values of thyrotropin? The values of the TSH are related tohypothalamus pituitary thyroid axis, an efficient self-regulating system of thyroid function. Thanks to this axis, the hormones fT3 and fT4 are kept in the normal range by the TSH secreted at the pituitary level, which in turn is regulated at the hypothalamic level by the secretion of TRH (thyrotropin releasing hormone). Very often, the finding of low TSH is consequent to a hyperfunction of the thyroid gland, and therefore to the presence of high values of fT3 and fT4. According to the hypothalamic-pituitary-thyroid regulation, in fact, the TSH secretion is regulated by the concentration of thyrotropin-releasing hormone, or TRH (its increase causes TSH production and secretion) and from the blood levels of fT3 and fT4 (when they increase, TSH secretion is reduced).
If there is therefore an overproduction of thyroid hormones, the pituitary gland tries to remedy it by decreasing the production of TSH, so that the thyroid is less stimulated to produce fT3 and fT4, returning to normal hormonal values. Initially this mechanism may be sufficient to compensate for the imbalance, but very often the continuous production of excess thyroid hormones leads to a TSH at almost indosable values and fT3 and fT4 constantly above the normal levels. This situation is called hyperthyroidism.
Why our TSH goes down? Below are the Common causes of low TSH:
Basedow-Graves disease: Graves' disease, also called diffuse toxic goiter, is an autoimmune disease caused by the presence of antibodies directed against the TSH receptor. In the long run, this can cause thyroid goiter formation with hormonal overproduction, and at the diagnostic level it will be characterized by hyperthyroidism with Low TSH, thyroxine and high triiodothyronine, and presence of TSH receptor antibodies.
Toxic nodular goiter: it is a benign neoplasm of the thyroid gland, characterized by a nodule (uninodular goiter or toxic thyroid adenoma, also called Plummer's disease) or several nodules (multi nodular goiter) that "work too much", producing large quantities of thyroid hormones . At the level of the blood analysis we will therefore have high fT3 and fT4 and low TSH.
Thyroiditis: it is an inflammatory process at the thyroid level, which can be of an infectious nature (such as acute or sub-acute thyroiditis) or autoimmune (such as chronic thyroiditis). In autoimmune thyroiditis there is a frequent finding of antibodies to thyroperoxidase and antibodies to thyroglobulin, and are characterized by Low TSH and high values of fT3 and fT4. The most frequent forms are Hashimoto's thyroiditis, postpartum thyroiditis and silent lymphocytic thyroiditis. In non-autoimmune thyroiditis (e.g. acute bacterial thyroiditis or De Quervain's subacute thyroiditis), anti-thyroid antibodies are found much less frequently.
Too high doses of hormone replacement therapy or other drug therapy: in patients already taking thyroid replacement therapy (i.e. taking thyroxine, the most commonly used trade name is Eutirox), a Low TSH with high thyroid hormones they may be the consequence of too high a dose of the drug. Even patients on lithium therapy for psychoactive therapy, or with amiodarone, for the control of cardiac arrhythmias, may have a hormonal imbalance in the hyperthyroid sense. In the case of amiodarone, high levels of free thyroxine are often found, with normal or low fT3. In this case we speak of low T3 syndrome, or a conversion defect from T4 to T3. Find out more about it Low TSH and Eutirox.
Ovarian teratoma: it is a neoplasm composed of embryonic tissues and therefore potentially also of thyroid type tissue, with ovarian localization. This tumor can also be benign, and causes hyperthyroidism for production not thyroid of T3 and T4, resulting in TSH reduction.
Familial non-autoimmune hyperthyroidism: this disorder is linked to the mutation of a gene that codes for the TSH receptor, and is transmitted in an inherited manner. The thyroid then escapes the pituitary hormone control, and begins to produce thyroid hormones in an uncontrolled way. As a result, the pituitary will decrease TSH production to attempt to limit the elevated levels of fT3 and fT4. We'll have blood tests then high thyroid hormones and low TSH.
Pregnancy: in the first three months of pregnancy, due to the increased levels of human chorionic gonadotropin (HCG), there is hyperstimulation of the thyroid gland, which results in a moderate increase in T3 and T4 and a slight decrease in TSH.
Hyperemesis gravidarum: it is a disorder that occurs during pregnancy, characterized by repeated vomiting episodes, which leads to even serious imbalances, often linked to unfavorable psychological conditions. In this situation, the increase in chorionic gonadotropin (HCG) stimulates the thorium to produce thyroid hormones. As a result, the pituitary gland will secrete lower levels of TSH. The biohumoral analyzes will therefore take place Low TSH and high fT3 and fT4.
Choriocarcinoma: it is a tumor of the trophoblast, which arises from the villi of the placenta. This rare form of neoplasm causes an increase in the concentration of chorionic gonadotropin (HCG) which, as in hyperemesis gravidarum, causes thyroid hyperstimulation with increased T3 and T4 and low TSH levels.
Subclinical hyperthyroidism: in the initial phase, hyperthyroidism occurs only with low thyrotropin values which they accompany normal levels of T3 and T4.
Dysfunctions of the pituitary gland: if the pituitary, despite good thyroid function, does not produce adequate levels of thyrotropin (TSH), the production of fT3 and fT4 will be negatively affected. In this case we speak of secondary hypothyroidism, characterized by low levels of TSH, fT3 and fT4 .
Dysfunction of the hypothalamus: it can rarely happen that the hypothalamus does not function properly, not producing adequate levels of TRH. Consequently, the pituitary is not adequately stimulated to produce the right amount of TSH. Again we will have Low TRH, low TSH and low fT3 and fT4.
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Low TSH, if not accompanied by elevated thyroid hormone values (therefore a condition of subclinical hyperthyroidism), often does not involve particular symptoms. If, on the other hand, there are high values of fT3 and fT4, it will manifest itself with the typical symptoms of hyperthyroidism, and therefore:
- Easy Irritability
- State of anxiety
- Presence of palpitations (presence of extrasystoles or ectopic beats, up to real arrhythmias)
- Tremor in the limbs
- Changes in the menstrual cycle
- Exophthalmos (eye protruding)
- Possible presence of eyelid swelling
- Hyperhidrosis (profuse sweating)
- Thinness and weight loss
- Hot flashes
In the less frequent case of low TSH levels associated with reduced fT3 and fT4, we may have the classic symptoms hypothyroidism, and therefore:
- Instability of mood
- Less tolerance to cold
- Weight gain
- Fragility of skin and nails
- Loss of eyebrows (especially the outer part)
- Constipation (constipation)
- Irregularity of the menstrual cycle,
- Increased blood levels of cholesterol
- Memory loss.
Low TSH in pregnancy
The finding of a Low TSH in pregnancy is often physiological: theTBG (Thyroxin Binding Globulin or thyroxine binding protein), which is none other than the transporter protein of thyroid hormones in the blood, due to the increased circulating levels of estrogen, in turn increases its plasma concentrations. The increased levels of circulating T3 and T4 exert a mild suppressive effect on TSH.
At the same time, the chorionic gonadotropin (hCG or human Chorionic Gonadotropin) produced by the placenta has an inhibiting effect on TSH production. This happens because hCG has a molecular structure that resembles that of the thyroid stimulating hormone and therefore acts on the thyroid TSH receptors activating the production of thyroid hormones, which in turn inhibit the release of TSH at the pituitary level. Here are the two reasons why physiologically is found Low TSH in pregnancy.
Having adequate levels of thyroid hormones circulating during gestation is important to ensure proper neurological and somatic development of the fetus. For this reason the thyrotropic hormone should be checked periodically, and the finding of Low TSH during pregnancy it must always be deepened with further analysis.
In fact, a percentage ranging from 0.1 to 3% of pregnancies may be subject, during the course, to the appearance of gestational hyperthyroidism, often associated with hyperemesis gravidarum. The other most frequent causes of hyperthyroidism in pregnancy I am the Graves' disease and toxic uninodular or multinodular goiter.
L'hyperthyroidism in pregnancy increases the risk of perinatal death, the risk of preterm birth or even miscarriage, the development of hypertension in pregnancy, and a low birth weight of the baby. It is also dangerous for the mother as it increases the incidence of maternal heart failure. For this reason hyperthyroidism during pregnancy should always be investigated to distinguish the cause and start the most appropriate therapy, checking the TSH every 2-4 weeks at the start of therapy, every 4-6 weeks after reaching the target value.
The examination is carried out starting from a blood sample obtained with a venous sampling. It is not necessary to fast before the exam. As some medications can interfere with TSH measurement, you should warn your doctor about all ongoing drug therapy.
Daily changes in blood TSH levels: the highest levels are found around 10 pm, while basal levels are measurable at 10 am.
Many drug therapies they can alter the TSH measurement. The following are the drugs that most commonly can lower TSH levels on blood tests:
- tricyclic antidepressants
- dobutamine and dopamine
Low TSH what to do
What happens in case of low TSH? What needs to be done?
First, the TSH must be considered together with the analysis of thyroid hormones and, if altered, of the antibodies anti-thyroperoxidase and thyroglobulin, in addition to the anti TSH receptor ab. Furthermore, thoroughness is essential anamnesis (i.e. the patient's history from childhood and evaluating nutrition, habits and present or previous diseases). A physical examination comprehensive to evaluate signs and symptoms and detect any warning lights of thyroid problems. All this can be done both from the general practitioner, as from one endocrinologist specialist.Tags: Laboratory medicine Blood