Thyroxine (T4) and its free fraction (FT4) represent more than 80% of the hormones produced by the thyroid. They are involved in the functioning of many vital organs. Their dosage makes it possible to identify a dysfunction of the thyroid, most often hyper or hypothyroidism.

Located on the front of the neck, the thyroid is an “endocrine” gland, ie it secretes hormones: among them, thyroxine or T4, produced from iodine and tyrosine (amino acid), which will give birth, after deionization to another thyroid hormone, triiodothyronine (T3).

L’hormone de la TSH

“These hormones are activated by stimulation of the thyroid by another hormone released by the brain, in the pituitary gland: TSH. Thus, the secretion of the thyroid hormones T4 and T3 is regulated in such a way that the quantity produced meets the body’s needs ”, explains Dr Deroide, Thyroid Surgeon.

The determination of T4 and its free fraction FT4

“The determination of T4 and its free fraction, FT4, is carried out by blood test at the request of the attending physician. It makes it possible to detect the onset of abnormal thyroid activity (hypothyroidism or hyperthyroidism) ”. The sample is taken when the TSH level is abnormal, to follow the progress of a treatment, monitor the adverse effects of a drug, determine the origin of symptoms that are difficult to explain (severe fatigue, muscle pain, anxiety) or in case of hypertension.

“Thyroid conditions are extremely common, especially in women, as it is estimated that nearly 40% of the female population (and 10% of the male population) have a thyroid abnormality. The majority only require simple monitoring, others medical treatment ”.

In adulthood, thyroid hormones are involved in the regulation of the heart, the intestine, in the production of heat, as well as in the metabolism (that is to say the transformation) of fats and sugars. “Thyroid hormones are the body’s fuel,” adds the surgeon. This is why if the thyroid does not produce enough hormones (hypothyroidism), it can appear constipation, chilliness, weight gain by water retention, memory lapses … On the other hand, if it does. product too much (hyperthyroidism) this will lead to palpitations, sweating, weight loss by melting fat and muscles, aggressiveness… ”.

In an adult aged 20 to 65 years, the norm for the level of T4 is between 80 and 140 nmol per liter of blood. These values ​​are much higher in newborns (120-210 nmol / L), decrease gradually during childhood then adolescence before stabilizing. It is sometimes the free fraction of thyroxine which is measured during the assay (FT4). The FT4 level should be between 8,6 and 25 pmol / L in adults.

Certain drugs can interfere with the dosage:

• thyroxine treatment,
• amiodarone,
• estrogen-progestins,
• heparin,
• corticosteroids,
• nonsteroidal anti-inflammatory drugs,
• salicylés,
• hydatoids, barbiturates.

T4 and FT4 levels may increase with hyperthyroidism, iodine overload, or anti-T4 antibodies without hyperthyroidism (increased FT4).

“The most common cause of hyperthyroidism in a young adult is Graves’ disease or also called exophthalmic goiter” continues the specialist. It is an autoimmune disease linked to a dysfunction of the immune system which will attack the thyroid ”.

In an older person, there is another form of hyperthyroidism which is the consequence of a toxic nodule or “toxic multinodular goiter”. “It is a nodule which has taken its independence vis-à-vis the thyroid and is no longer regulated by the body TSH, summarizes the specialist. It thus permanently manufactures thyroid hormones and induces a drop in TSH ”.

Low levels of T4 and FT4 can help diagnose hypothyroidism, iodine deficiency, or extra-thyroid disease.

• if the free T4 (fT4) is normal, we speak of rough (or subclinical) hypothyroidism and the TSH is then low, most often between 4 and 10 mIU / L;
• if the free T4 (fT4) is low, it is called overt hypothyroidism and the TSH is then higher, greater than 10 mIU / L.

“Hypothyroidism affects women and the elderly more frequently, those who have a personal or family history of thyroid disease or autoimmune disease (type 1 diabetes, celiac disease, etc.), informs the doctor. It also concerns patients who no longer have a thyroid ”.

Hashimoto’s thyroidite

Hypothyroidism can also be linked to Hashimoto’s thyroiditis. “This chronic autoimmune disease, characterized by high levels of antithyroid antibodies in the blood, will lead to cell destruction of the thyroid, and a release of thyroid hormones.”