Endocrine diseases

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Endocrine diseases
portrait of Fernando Martínez Sáez
Written by

Fernando Martínez Sáez
Medically reviewed by our Medical staff

Last update: 16-09-2021

How else can it be called?

  • Underactive thyroid

  • ICD-10: E03

What is hypothyroidism?

Hypothyroidism is an endocrine disorder resulting from a deficiency of thyroid hormones in the blood. This is usually due to an underactive thyroid gland that does not release enough thyroid hormone.

How are thyroid hormones produced?

Thyroid hormones are produced in the thyroid gland. The thyroid gland is located in the front part of the neck, under the Adam's apple. It has a butterfly shape and wrapped around the windpipe (trachea). The thyroid gland has two similar-sized lobes joined by an isthmus across the front of the trachea.


Figure 1. Thyroid

Thyroid hormones are essential for the cells. They play an important role in growth and development, heart rate, blood pressure, body temperature, and metabolic rate (the rate at which food is converted into energy in the body cells).

The thyroid gland uses iodine to produce thyroid hormones.

The 2 most important thyroid hormones are:

  • Thyroxine (T4)
  • Triiodothyronine (T3).

The T4 hormone contains 4 iodine molecules, while the T3 hormone only contains 3 iodine molecules.

The iodine needed to produce thyroid hormones is found in fish and mollusks, bread, and iodized salt.

More than 99% of all thyroid hormones are bound to proteins in the blood and are inactive, meaning they cannot interact with the body's cells. Only a small fraction of circulating thyroid hormones are free, unbound to proteins. This small fraction of thyroid hormones is essential for the regulation of cellular metabolism.

The production of thyroid hormones is controlled by the release of a hormone called TSH (Thyroid Stimulating Hormone) produced by the pituitary gland or hypophysis. This gland is located at the base of the brain.

When there is an excess of thyroid hormones in the blood, the pituitary gland stops the secretion of TSH, and vice versa. This helps maintain a relatively constant level of circulating thyroid hormones in the blood.

What types of hypothyroidism are there?

When a person suffers from hypothyroidism, the concentration of thyroid hormones T3 and T4 in the body decreases.

However, depending on the reason of the decreased production of thyroid hormones, hypothyroidism can be classified into:

  • Primary: The thyroid gland is not working effectively. It is the most common cause (95% of cases). The thyroid gland is not able to produce enough thyroid hormones T3 (triiodothyronine) and T4 (thyrotropin). In a blood test, T3 and T4 levels are below the normal range, while TSH level is above the normal range.
  • Secondary or pituitary: The pituitary gland or hypophysis is not working properly. It may be caused by a tumor in the pituitary gland that prevents the secretion of the TSH hormone. In a blood test, T3, T4, and TSH levels are low.
  • Tertiary or hypothalamic: With the origin in the hypothalamus. This type is extremely rare. It is normally due to the inability to produce TRH (thyrotropin-releasing hormone) that regulates TSH hormone levels. In a blood test, TRH level will be below the normal range.

Secondary and tertiary hypothyroidism is usually group together and called central hypothyroidism.

What are the main causes of hypothyroidism?

The main underlying causes that causes hypothyroidism are:

  • Iodine deficiency: Severe hypothyroidism may occur in areas or countries where there is an iodine deficiency in the diet, such as the Congo, India, Ecuador, Chile, or in remote mountain areas such as the Andes and the Himalayas. Severe hypothyroidism affects 5-15% of people in these countries. To prevent this deficiency, iodine is often added to table salt.
  • Thyroiditis: In developed countries, the most common cause of hypothyroidism is an autoimmune inflammation of the thyroid gland called chronic lymphocytic thyroiditis or Hashimoto's thyroiditis (named after Dr. Hakaru Hashimoto, who first described it in 1912). This disease is 5-10 times more common in women. There are very high levels of antithyroid antibodies (as can also occur in Graves' disease). Some types of thyroiditis (after pregnancy or after suffering from a viral disease) can lead to transient states of hypothyroidism.
  • Thyroidectomy: Thyroid surgery or ablation with radioactive iodine may cause hypothyroidism.
  • Drugs: In addition to antithyroid drugs, lithium (used in mental disorders) may produce hypothyroidism. Occasionally, certain iodine rich-drugs can also produce hypothyroidism, such as amiodarone, an antiarrhythmic agent.
  • Congenital hypothyroidism (cretinism): Children born without a thyroid gland (cretinism) may develop mental and growth disabilities. This can be avoided if the condition is detected and treated early. Therefore, thyroid hormones are measured in a routine blood test to all newborns.

What incidence does it have?

Hypothyroidism is the most common thyroid disease. It affects 3-5% of the general population.

Women are more likely to suffer hypothyroidism than men with a female-to-male ratio of 6 to 1.

What are the main symptoms of hypothyroidism?

Patients with mild hypothyroidism may be asymptomatic. Symptoms become more evident as the disease develops. They usually produce a metabolism slowdown.

The most common symptoms are:

  • Fatigue
  • Weakness
  • Cold intolerance
  • Constipation

Other symptoms relate to hypothyroidism may include:

  • Lethargy
  • Weight gain
  • Hair loss
  • Depression
  • Increased cholesterol levels

What is the recommended treatment?

Hypothyroidism can be successfully treated. However, if left untreated, it can lead to serious heart disease (cardiomyopathy, heart failure), as well as a very serious situation called myxedema coma. Stress, concomitant diseases, or surgery in a previous severe hypothyroid state usually triggers myxedema coma. It must be treated in a hospital with intravenous thyroid hormone therapy.

Furthermore, the treatment of hypothyroidism is based on the replacement of thyroid hormones with the T4 (thyroxine) synthetic hormone called levothyroxine. The daily dose is usually 100-150 micrograms (0.100 to 0.150 milligrams). After six weeks of treatment, thyroid hormones (T3 and T4) and TSH hormone levels should be measured.

The ultimate goal of the treatment is to maintain a normal level of TSH, which is the main priority (a normal level of thyroid hormones will be the other objective). Overtreatment of hypothyroidism is as dangerous as not treating it.

Medically reviewed by our Medical staff on 16-09-2021


  • First Aid for the Basic Sciences: Organ Systems (3rd Ed) 2017, Tao Le, William L. Hwang, Vinayak Muralidhar, Jared A. White and M. Scott Moore, ISBN: 978-1-25-958704-7, Pag. 126.
  • Oxford Handbook of Clinical Immunology and Allergy (3rd Ed) 2013, Gavin P Spickett, ISBN: 978–0–19–960324–4 Pag. 171.
  • Robbins Basic Pathology (10th Ed) 2018, Vinay Kumar, Abul K. Abbas, Jon C. Aster, ISBN: 978-0-323-35317-5, Pag. 757.
  • Hypothyroidism. American Thyroid Association. Available on: https://www.thyroid.org

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