Thyroid hormones: types and functions in the human body

The endocrine system is made up of all those glands and hormones that act in our body. Hormones play a very important role in regulating 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 its origin, its characteristics and its functions. In addition, we will analyze two important alterations that occur in the thyroid: hyperthyroidism and hypothyroidism.

• Related article: "Types of hormones and their functions in the human body"

Thyroid hormones: characteristics

Thyroid hormones are a type of hormones secreted in our body, and with 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.

For its part, metabolism 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 the 20 amino acids that make up proteins). Specifically, thyroid hormones are amino hormones, along with other hormones: adrenaline, norepinephrine, melatonin, and dopamine. Interestingly, these latter substances are in turn neurotransmitters (within the Central Nervous System [CNS]) and hormones (outside of it).

But how do amino hormones work? What they do is bind to receptors on the cell membrane, starting a chain reaction in the cell. Let's see 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.

At the functional level, thyroxine what it does mainly is stimulate the body's metabolism, in addition to participating in other processes. It is important, for proper functioning, that thyroxine levels are adequate and are balanced, as too high or too low levels can cause disturbances throughout the entire organism.

This is what happens when thyroid disorders appear: hyperthyroidism (increased hormones thyroid) and hypothyroidism (decreased thyroid hormones), which we will explain later with 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 it also has a central role in the control and regulation of body metabolism. What it does is stimulate the metabolism of carbohydrates and fats, through the activation of oxygen consumption.

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

• You may be interested: "Parts of the Nervous System: anatomical structures and functions"

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 "commands", connecting the nervous system with the endocrine system. This, in turn, causes 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 hypothalamus posterior (or neurohypophysis). The neurohypophysis is the "storage organ" for these hormones.

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

That is, thyroid hormones (thyroxine and triiodothyronine) come from trophic hormones, which in turn come from the anterior pituitary. Specifically, thyroid hormones are stimulated by TSH and thyrotropin, a type of trophic hormone. By way of synthesis, what these structures (TSH and thyrotropin) actually do is stimulate the release of thyroid hormones in the thyroid gland.

Thyroid gland

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

It is not a very large 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. What's more, 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 the following:

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

Alterations

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

1. Hyperthyroidism

Hyperthyroidism consists of an elevated secretion of thyroid hormones; specifically, secretes too much thyroxine. That is, the thyroid becomes overactive and as a result, the body's metabolism accelerates.

This causes significant weight loss., as well as fast and / or irregular heartbeats. It is related to symptoms of hyperarousal and mania (manic episodes of euphoria and overexcitement). Other common symptoms are irritability, mood swings, fatigue, muscle weakness, and trouble sleeping.

Hyperthyroidism affects women more often than men. Another particularly affected population is people over 60 years of age.

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

2. Hypothyroidism

Hypothyroidism would be the opposite alteration; it implies a poor secretion of thyroid hormones. Specifically, the thyroid does not secrete enough thyroxine to be able to develop normal body functions.

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

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

Bibliographic references:

• Carlson, N.R. (2005). Physiology of behavior. Madrid: Pearson Education.
• Netter, F. (1989). Nervous system. Anatomy and physiology. Barcelona: Salvat.