The thyroid is a butterfly-shaped endocrine gland located in the lower anterior region of the neck, in front of the trachea. It is composed of two portions called lobes, the right lobe and the left lobe, connected to each other by a thin flap of thyroid parenchyma, called the isthmus. The main role of the thyroid gland consists in the production of the thyroid hormones, triiodothyronine (T3) and thyroxine (T4), which are secreted into the blood and subsequently transported to the body’s tissues. Thyroid hormones help regulate body metabolism, body temperature and maintain proper functioning of the brain, heart, muscles and other organs.
How does the thyroid gland work?
The thyroid, like other endocrine glands, is regulated by a gland located at the base of the brain, the pituitary, which produces, among others, the thyrotropic hormone (or thyroid stimulant) known by the acronym TSH (from the English Thyroid Stimulating Hormone).
TSH stimulates the thyroid to produce the thyroid hormones T3 and T4. In the periphery, T4 is converted by the deiodinase enzyme into T3, the most active hormonal form.
Thyroid hormones circulate in the blood bound to transport proteins and only the small part of them that remains free from binding to proteins, the so-called fT3 and fT4 hormones (free T3 and free T4, from the English “free” which means “free ”), Is responsible for the hormonal activity on the various target tissues.
What tests are done to study the thyroid gland?
The basic tests to evaluate the thyroid gland are:
Blood tests: The TSH it is the most sensitive hormone to evaluate the functioning of the thyroid gland.
In most cases, a normal TSH value indicates that thyroid function is normal (euthyroidism).
- The reflex TSH: when the reflex TSH value is within the normal limits, fT4 is not measured.
- When the blood concentration of the reflex TSH is altered (increased or reduced), also fT4 is automatically dosed. The fT3 value must therefore be specifically requested in order to complete the picture of thyroid function.
- In case of increased TSH with reduced fT3 and fT4 it is diagnosed hypothyroidism, while in the case of reduced TSH with increased fT3 and fT4 hyperthyroidism is diagnosed.
- There are also more complex cases, such as the T3-toxicosis, in which TSH is suppressed, fT4 is low, but with high circulating levels of fT3. In cases like this, the TSH reflex is not enough. Combined measurement of TSH, fT3, fT4 is required.
- Calcitonin:hormone produced by C cells, also called parafollicular. It increases in medullary thyroid cancer, a rare subtype of thyroid cancer.
- Thyroid antibodies: the thyroid is a gland that is often subject to attack by the immune system. Thyroid autoimmunity means that the immune system recognizes as foreign what is not foreign: the thyroid gland.
- The antibodies of clinical interest are those anti-thyroglobulin (AbTg) and anti-thyroperoxidase (AbTPO),which arise more frequently in Hashimoto’s chronic autoimmune thyroiditis, and the TSH receptor antibodies (TrAb),which cause super-stimulation at the thyroid level, outlining a clinical picture known as Graves-Basedow’s disease.
Thyroid ultrasound
The thyroid ultrasound allows to study, through the use of ultrasound, the morphology of the gland. In chronic thyroiditis and Graves-Basedow’s disease, ultrasound shows a rarefaction of the echo structure due to autoimmune lymphocyte infiltration. Ultrasound allows you to view the characteristics of the thyroid nodules and to stratify the risk of cancer.
Thyroid scintigraphy
Thyroid scintigraphy has lost importance in the morphological study of the thyroid, but retains some indications in the functional study, in particular
- Hyperfunctioning / hypercaptating nodules (so-called warm nodules)
- Hyperfunctioning multinodular goiter. Scintigraphy allows to visualize hypercaptive areas. This is useful in the diagnostic setting of thyroid nodules.
- Hypercaptant (warm) nodules are only exceptionally malignant.
What are the most common diseases of the thyroid gland?
- Hypothyroidism: reduced thyroid function
- Hyperthyroidism: increased thyroid function
- Thyroid nodules: lesions within the thyroid
- Goiter: an increase in the volume of the thyroid gland
- Thyroiditis: inflammation of the thyroid gland
