What is Hypothyroidism? Everything You Need to Know

· September 18, 2017
What is hypothyroidism? Learn all about this condition and its biological causes in this article.

What is hypothyroidism?

Hypothyroidism is an endocrine system disease. The term “hypothyroidism” describes the set of clinical signs that result in a reduced thyroidal hormone action in your tissues.

In this article, we’ll take a look at the biological causes of this condition.

The function of the hypothalamus-pituitary-thyroid axis

TSH (thyrotropin, a Thyroid-stimulating hormone) is produced in the anterior pituitary gland after it receives the hypothalamic hormone TRH (Thyrotropin-releasing hormone).


TSH attaches with specific receptors. These are located in the cells in your thyroid. In response, this starts a series of processes whose final result is the creation and secretion of thyroid hormones.

  • T4 (Thyroxine) is only made in your thyroid.
  • About 20% of T3 (Triiodothyronine) is created in your thyroid. The other 80% is made in the target tissue using T4. The process is carried out by a deiodinase enzyme.
  • Part of the T4 hormone turns into its inactive form, T3r. This is also performed by a deiodinase enzyme.

It is regulated through a feedback mechanism. When the concentration of T3 and T4 reaches its physiological value, they inhibit the production of TSH and TRH. In other words, they stop stimulating their secretion.

What is the main cause of hypothyroidism?

The main cause is the reduction in hormonal secretion. This is what happens in most cases. According to the anatomical level where this change was caused, hypothyroidism can be classified as primary, secondary, or tertiary.

Primary hypothyroidism

In this case, the change happens in the thyroid itself. The gland reduces its production of thyroid hormones for the following reasons:

  • In light of a loss of glandular mass. This reduces both the number of glandular cells and the amount of hormones that they produce. This is what happens in chronic lymphocytic thyroiditis (the main cause of adult hypothyroidism). It can also happen as a result of treatment with radioactive iodine or after a thyroidectomy (the total or partial removal of the thyroid).
  • Because of a congenital defect in thyroidal development.
  • Because of an iodine insufficiency in a diet. This is the main cause of hypothyroidism in developing countries.
  • Due to a block in the production of thyroid hormones. The process of thyroid hormone production can change at any of its steps because of either internal or external causes. When it is from an internal origin, the change is due to a deficiency during the process. When it comes from an external source, most of the time it is due to the consumption of certain medications (lithium salts).

Since the hypothalamic–pituitary–thyroid axis follows a negative feedback mechanism, a decline in thyroid hormones results in an increase in TSH (which eventually increases thyroid hormones).

The increase of TSH above normal levels is the most sensible analytical data for diagnosing primary hypothyroidism.

Secondary hypothyroidism

This is caused by a reduced secretion of thyroid hormones as a result of a reduced production of TSH in the pituitary gland. By reducing the secretion of TSH below normal levels, this under-stimulates the thyroid (which functions correctly). The final result is a reduced production of T3 and T4.

 This can be due to:

  • An ischemic necrosis of the gland (it dies due to lack of blood flow).
  • The presence of tumors.
  • A general failure of the anterior hypothalamus. This is known as Sheehan’s syndrome.
  • Due to a negative feedback mechanism that regulates this axis. The result is an increase in the TRH hormone (to stimulate the secretion of TSH). However, this increase doesn’t have any effect on the hypothalamus because it is the source of the failure.

In other words, in secondary hypothyroidism, there isn’t a response to an increase in TRH.

Tertiary hypothyroidism

An underactive thyroid, in this case, is due to a reduction in the secretion of TRH in the hypothalamus. By reducing the stimulation of the hypothalamus, it secretes a reduced amount of TSH. As a result, this also reduces thyroidal stimulation. This then causes a reduction in the secretion of T4 and T3.

In this case, even though there is a reduction in thyroidal hormones, there isn’t an increase in the hypothalamus’s TRH. However, the defect can be corrected through an exogenous administration of TRH given that the pituitary gland responds to it.

Tissues’ peripheral resistance to hormones 

This happens on rare occasions. This produces mutations in your genes related to the thyroid receptors that are present in tissues. As a result, these receptors change their normal functions. This causes the tissues to not respond to the hormones.

In other very rare cases the problem can be due to the peripheral inactivity of thyroid hormones. This can have to do with the changes in the conversion processes of T4 to T3 and of T4 to T3r. This inhibits the transformation of T4 to T3. This causes a reduction of T3 in the analysis. It also increases the T3r, which is the inactive form of the hormones.

Clinical signs of hypothyroidism

On the one hand, symtpoms are related to a hormonal deficit. This translates to a slowing of the metabolic processes. This includes:

  • A constant feeling of being cold despite the ambient temperature.
  • Pale, cold, rough skin. Skin, eyebrows, and eyelashes that become fragile and tend to fall out.
  • Slowing heartbeat and hypertension.
  • Constipation caused by a slowing intestine.
  • Reduced concentration and changes to language and memory. The patient finds themselves moving slower. In cases of bad control and large changes, a kind of coma called myxedema appears.
  • Goiters appear in cases of insufficient iodine. They also appear if there’s a lack of the production of T3 and T4.
  • Anemia and hypocholesterolemia.
  • In children: stunted growth.

In addition, there is a series of signs that appear from the mucoid tissue deposit. This is composed of water and hyaluronic acid. This is deposited in the tissues as a result of a weakened metabolism of polysaccharides

  • Facial swelling that can spread to your extremities. The swelling that’s caused by hypothyroidism received the name myxedema.
  • Weight gain.
  • Increase in tongue size and a hoarse voice.
  • Carpel tunnel syndrome.

How is it diagnosed?

Analysis. This allows the doctor to determine the thyroidal hormones T3 and T4 as well as the concentration of TSH.


  • Reduction of free T3 and T4 and increase in TSH: primary hypothyroidism
  • Below normal levels of T3 and T4 + reduced TSH without a response to external TRH: Secondary Hypothyroidism
  • Reduction of T3 and T4 + reduced TSH that responds to external TRH: Tertiary Hypothyroidism

Morphological studies with a scintigraphy, radioactive iodine, and a thyroid sonogram

Treatment of hypothyroidism

The treatment of hypothyroidism consists of replacing the thyroidal hormone T4, thyroxine. The most used medicine is levothyroxine. Its prescription should use the smallest doses possible to reestablish normal levels of T4.

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