**Index**
**Thyroid hormones are among the elements that can be assessed with a** blood test.
**Thus, thyroid hormone tests allow the assessment of thyroid function, a gland located below the larynx and in front of the trachea, which produces hormones crucial for controlling** **metabolism**.
**What are thyroid hormones**
**Thyroid hormones are hormonal substances that regulate metabolism and have a direct impact on caloric expenditure and a multitude of associated factors, such as** **weight gain****, etc.**
**In a blood test, T levels are usually measured**3 **(triiodothyronine) and T**4 **(thyroxine). Most of these thyroid hormones circulate in the blood bound to transport proteins. Only a fraction circulates freely, the free fraction. It is this free fraction that acts on the tissues and organs of the body.**
**Functioning of the thyroid gland**
**As with other endocrine glands in the body, the functioning of the thyroid gland is in turn regulated by the** **pituitary gland****, a gland located below the brain, which produces thyroid-stimulating hormone (better known by its English acronym,** **TSH**).
**When the thyroid produces too few hormones, the pituitary gland produces more TSH to stimulate its function. Conversely, when the thyroid produces an excessive amount of hormones, the pituitary releases less TSH. This type of regulation, or feedback, is characteristic of hormonal systems. Knowing the levels of thyroid hormones and TSH thus provides a fairly accurate idea of the functioning of the thyroid gland.**
**Thyroid hormone levels**
**T levels are considered normal**3 **total between 70 and 190 ng/dL (nanograms per deciliter) and T**4 **total between 5 and 12 µg/dL (micrograms per deciliter).**
**Hyperthyroidism**
**When thyroid hormones are produced in excessive amounts, hyperthyroidism occurs. In this way, the test will show elevated levels of thyroid hormones.**
**Hyperthyroidism is characterized by an accelerated metabolism. Possible causes include Graves' disease (an autoimmune disorder), hyperfunctioning thyroid nodules, and thyroiditis (inflammatory diseases of the thyroid).**
**Hypothyroidism**
**Conversely, when thyroid hormones are produced in insufficient amounts, hypothyroidism occurs.**
**Low levels of thyroid hormones are characterized by a decreased metabolism. This can be due, for example, to Hashimoto's thyroiditis (an autoimmune disease), or damage to the thyroid from surgery or radiotherapy. Worldwide, the leading cause of hypothyroidism is severe iodine deficiency, but in developed countries it is uncommon.**
**Epidemiology of hypothyroidism and hyperthyroidism**
**Globally,** **some estimates** **indicate a prevalence of hyperthyroidism ranging from 0.2% to 1.3% of the population in regions with adequate iodine intake. Similarly, the prevalence of hypothyroidism is estimated to be around 1–2%.**
**High TSH**
**Normal TSH values range from 0.4 to 5 mIU/L (milli-international units per liter). An elevated TSH indicates the presence of primary hypothyroidism, in which the thyroid gland does not produce enough hormones, causing the pituitary gland to produce excess TSH to counteract this condition.**
**Conversely, a decrease in TSH indicates primary hyperthyroidism.**
**However, abnormal TSH values should be interpreted together with thyroid hormones and in the context of each patient.**
**Pituitary gland disorder**
**Although less frequently, a dysfunction of the pituitary gland can occur, in which case thyroid hormones will vary in parallel with TSH.**
**Additionally, certain physiological conditions or treatment with some medications can slightly alter the plasma levels of these hormones without necessarily indicating the presence of disease.**
**Anti-thyroid antibodies**
**A final test that can be performed in relation to thyroid function is the measurement of the so-called** **anti-thyroid antibodies****, which the body produces against the thyroid gland in cases of autoimmune diseases.**
**This is a different type of test from that of hormones. The presence of these antibodies may indicate an autoimmune disease, such as Graves' disease or Hashimoto's thyroiditis.**