The endocrine system is formed by all those glands and hormones that act in our body. Hormones play a very important role in the regulation of basic physiological processes; in addition, they are also especially related to emotions.

In this article we will talk about thyroid hormones , a type of hormones synthesized by the thyroid gland and involved in metabolism. We will know their origin, their characteristics and their functions. In addition, we will analyze two important alterations that occur in the thyroid: hyperthyroidism and hypothyroidism.

Thyroid Hormones: Characteristics

Thyroid hormones are a type of hormones secreted in our body, and have important functions. Specifically, there are two: thyroxine (T4) and triiodothyronine (T3) . These hormones are produced by the thyroid gland, a very important gland that regulates the body’s metabolism.

The metabolism, on the other hand, is involved in the processes that control the rate of activity of different cells and tissues, and encompasses a series of biological and chemical changes that occur continuously in the cells of our body.

Thyroid hormones are based on tyrosine (one of 20 amino acids that make up proteins). Specifically, thyroid hormones are amine hormones, along with other hormones: adrenaline, noradrenaline, melatonin and dopamine. Interestingly, the latter substances are both neurotransmitters (within the Central Nervous System [CNS]) and hormones (outside of it).

But how do the amine hormones work? What they do is bind to receptors on the cell membrane, initiating a chain reaction in the cell. Let’s look at the characteristics of the two thyroid hormones:

1. Thyroxine (T4)

Thyroxine was discovered and isolated in 1910 by Edward Calvin Kendall, an American researcher. Specifically, he isolated this substance from the thyroid of pigs.

On a functional level, thyroxine mainly stimulates the body’s metabolism , in addition to participating in other processes. It is important, for proper functioning, that thyroxine levels are adequate and balanced, as levels that are too high or too low can cause alterations throughout the body.

This is what happens when alterations in the thyroid appear: hyperthyroidism (increase in thyroid hormones) and hypothyroidism (decrease in thyroid hormones), which we will explain later in detail.

2. Triiodothyronine (T3)

The second of the thyroid hormones, triiodothyronine, was discovered 42 years later than thyroxine, in 1952, by Jean Roche, a French biochemist.

This hormone also plays a central role in the control and regulation of the body’s metabolism . What it does is stimulate the metabolism of carbohydrates and fats, through the activation of oxygen consumption.

In addition, triiodothyronine is also involved in different physiological processes in the body, such as growth, heart rate and body temperature (just like thyroxine). Finally, another function it performs is to degrade the proteins inside the cells.

Where do thyroid hormones come from?

To understand where thyroid hormones come from, we must visualize a global scheme of hormones and the endocrine system. The endocrine system is headed by the hypothalamus , the main secretor of hormones and the structure that “rules”, connecting the nervous system with the endocrine system. The endocrine system, in turn, produces two types of hormones: releasing hormones on the one hand, and oxytocin and vasopressin on the other.

While the former (releasing hormones) act on the anterior hypothalamus (or adenohypophysis), the latter (oxytocin and vasopressin) act on the posterior hypothalamus (or neurohypophysis). The neuro-pituitary is the “storage organ” for these hormones.

More specifically, the adenohypophysis manufactures the trophic hormones, which in turn act on the glands; these produce the different hormones in the body. This is what happens with the thyroid hormones: which are synthesized by the thyroid gland, which in turn receives signals from the adenohypophysis (a part of the hypothalamus, the anterior part).

That is, the thyroid hormones (thyroxine and triiodothyronine) come from the trophic hormones, which in turn come from the adenohypophysis. Specifically, thyroid hormones are stimulated by TSH and thyrotropin, a type of trophic hormone. In synthesis, these structures (TSH and thyrotropin) actually stimulate the release of thyroid hormones into the thyroid gland.

Thyroid gland

The thyroid gland, or thyroid, is the structure that secretes thyroid hormones (it is actually an organ). This endocrine gland, which is shaped like a butterfly, is located in the neck area , just above the collarbone and below the walnut.

It’s not a very big structure, and it weighs about 30 grams. The thyroid plays an essential role in our body’s metabolism, as well as in other body functions such as body temperature. Furthermore, its condition and functioning are closely related to our state of health .

Some of the functions in which the thyroid gland is involved, through the action of its thyroid hormones, are as follows:

  • Participation in growth.
  • Regulation of the metabolism.
  • Regulation of body temperature
  • Development of the nervous system.
  • Assimilation of nutrients.
  • Heart rate regulation.
  • Skin development.

Alterations

There are two major alterations in the thyroid gland , which affect its secretion of thyroid hormones: hyperthyroidism and hypothyroidism.

1. Hyperthyroidism

Hyperthyroidism consists of an elevated secretion of thyroid hormones; specifically, secretes too much thyroxine . In other words, the thyroid is overactivated and as a result, the body’s metabolism is accelerated.

This causes significant weight loss , as well as rapid and/or irregular heartbeat. It is related to symptoms of hyperactivity and mania (manic episodes of euphoria and over-excitement). Other common symptoms are irritability, mood swings, fatigue, muscle weakness, and sleep problems.

Hyperthyroidism affects women more often than men. Another population that is especially affected is people over 60.

Its causes can be diverse; the most common cause is Gave’s disease , a type of autoimmune disorder. Other possible causes are thyroiditis, overuse of iodine, or having thyroid nodules.

2. Hypothyroidism

Hypothyroidism would be the opposite disorder; it involves a poor secretion of thyroid hormones. Specifically, the thyroid does not secrete enough thyroxine to be able to carry out the functions of the body normally .

This implies alterations in the metabolism, which is deficient; thus, the person with hypothyroidism presents a weight gain (gets fat easily), and also manifests depressive symptoms, fatigue and swelling in the face, among others. Hypothyroidism, like hyperthyroidism, affects more women than men.

The causes of hypothyroidism can be several; among them, there is a genetic form called “hereditary botrytistic cretinism,” in which a deficit of thyroid hormones causes severe growth retardation, facial malformations, alterations in sexual development, and a decreased brain size and number of synaptic connections. This deficit of thyroid hormones also causes intellectual disability.

Bibliographic references:

  • Carlson, N.R. (2005). Behavioral physiology. Madrid: Pearson Educación.
  • Netter, F. (1989). Nervous system. Anatomy and physiology. Barcelona: Salvat.