The thyroid hormones are secreted by the thyroid, a gland located in the neck, and are produced from iodine and an amino acid (the tyrosine). They are called triiodothyronine (T3) and thyroxine (T4), and are under the control of a pituitary hormone called TSH (Thyroid Stimulating Hormone) or thyroid stimulating hormone. When thyroid hormone levels decrease, TSH levels increase and vice versa.

T4 represents about 80% of the hormones produced by the thyroid. It is an inactive hormone, but it can convert to T3, which it is active, after deionization in the target cells. It is said that T4 is the pro-hormone of T3.

Thyroid hormones are linked in high proportion to transport proteins in the bloodstream: TBG, TBPA, or even albumin.

Thyroid hormones affect all tissues by participating in the regulation of many metabolic reactions. Let us quote for example:

• growth and development of tissues (development of the central nervous system, production and renewal of blood cells (hematopoiesis), etc.)

• nutrient metabolism

• thermoregulation

• regulation of the cardiovascular system

If the doctor suspects a thyroid disorder, he may prescribe a TSH dosage and thyroid hormones to confirm and assess the extent of the dysfunction.

First, to confirm or not the presence of a thyroid involvement, the doctor prescribes a test for TSH in the venous blood. To do this, it suffices to take a blood test, generally at the level of the elbow crease.

If the TSH values ​​are abnormal, and there is therefore a suspicion of a thyroid disorder, the doctor may subsequently prescribe a T4 and / or T3 test to assess the extent of the thyroid dysfunction. The dosage is also done from a venous blood sample but it is not systematic.

The thyroid hormone assays make it possible to demonstrate a hyperthyroidism or a hypothyroidism (excessive or on the contrary insufficient secretion of thyroid hormones).

Le TSH level is normally between about 0,2 and 4 mU / L (milli-international units per liter). If this is indeed the case, the thyroid is considered to be functioning properly. A high value indicates a hypothyroid, while an abnormally low value is usually sufficient to diagnose hyperthyroidism.

To refine his diagnosis, the doctor can ask for the dosage of free T4. This will allow him to confirm hyper or hypothyroidism and assess its importance. The higher the rate, the more pronounced the hyperthyroidism. The lower it is, the more hypothyroidism is proven.

The determination of free T3 is more sensitive and specific than that of free T4, but it is more difficult to perform, the concentrations being low. It is therefore less commonly used.

The doctor may also order a scan of the thyroid, by ultrasound, to assess its condition and size.

If the patient has hypothyroidism, they may have the following symptoms:

• weight gain

• dry skin

• Constipation

• cold intolerance

• fatigue

• hair loss

• presence of a goiter, i.e. an increase in the size of the thyroid gland

• or irregular menstruation in women

In case of hyperthyroidism, the symptoms are for example:

• increased heart rate

• anxiety

• weight loss

• difficulty sleeping

• light sensitivity

• or even tremors in the hands