Low thyroid function (hypothyroidism)

Hypothyroidism is a disorder characterized by the decreased production of the thyroid hormones.

Thyroid gland and doctor. Doctor in white medical lab coat with stethoscope around his neck shows the thyroid gland modelOverview

Hypothyroidism is a common condition characterized by the insufficient production of thyroid hormones – T3 (triiodothyronine) and T4 (thyroxine). As long as thyroid hormones regulate the body’s metabolism their deficiency reduces the speed of the chemical reactions and processes occurring in the body, therefore, almost all body systems are involved. The severity of the disorder varies from mild to a condition known as myxedema coma.

The incidence of hypothyroidism is somewhat higher among women and in the elderly and in general approximately 1% of the world’s population is affected.

Causes and classification

  1. Primary hypothyroidism is caused by the decreased secretion of the thyroid hormones by the thyroid gland. In about 99% of cases, hypothyroidism occurs due to thyroid gland related issues.
    • Acquired
      • Iodine deficiency (endemic goiter);
      • Autoimmune destruction of the thyroid gland – Hashimoto thyroiditis, atrophic thyroiditis;
      • Post-thyroiditis hypothyroidism;
      • Radiation injury;
      • Resection of the thyroid gland;
      • Infiltration of the thyroid gland due to amyloidosis, hemochromatosis, sarcoidosis, scleroderma, etc.;
      • Some drugs, including iodine excess (amiodarone), lithium, aminosalicylic acid, etc.;
    • Congenital
      • Thyroid agenesis or dysplasia (complete absence or reduced volume of the thyroid gland);
  1. Central hypothyroidism is defined as the decreased production of the thyroid hormone in response to the insufficient secretion of the hormones which boost the activity of the thyroid gland by the hypophysis or hypothalamus when these organs are damaged by the tumor, trauma or infiltrative diseases (sarcoidosis, cystinosis, hemochromatosis, progressive systemic sclerosis, amyloidosis, Riedel’s thyroiditis with fibrosis).
    • Secondary hypothyroidism occurs due to hypopituitarism when not enough TSH is being released by the pituitary gland;
    • Tertiary hypothyroidism due to the hypothalamus dysfunction when the concentration of TRH (thyrotropin-releasing hormone) is too low;
    • TSH resistance is observed when the thyroid gland receptors, which bind TSH molecules are resistant to TSH;

Risk  factors

The risk of developing hypothyroidism is higher in individuals who:

  • Are older than 60 years;
  • Have autoimmune diseases such as lupus, rheumatoid arthritis, etc.;
  • Have a family history of thyroid disorders;
  • Live in iodine-deficient, endemic regions;
  • Were treated with radioactive iodine or anti-thyroid medications;
  • Received radiotherapy applied to the neck or upper chest;
  • Had been previously operated – had a partial thyroidectomy (removal of the part of the thyroid gland);


Most symptoms of hypothyroidism are non-specific and depend on the severity of the disease and the extent of the hormone deficit. Symptoms of low thyroid function include:

  • Puffy face and eyelids;
  • Pale skin;
  • Tiredness, weakness;
  • Sleepiness;
  • Dry skin;
  • Feeling cold (cold intolerance);
  • Hair loss and thinning;
  • Poor memory;
  • Constipation;
  • Weight gain with poor appetite;
  • Muscle pain and cramping;
  • Joint pain;
  • Hoarse voice;
  • Paresthesia, compression of the median nerve on the wrist (carpal tunnel syndrome) leading to the pain, numbing and tingling sensation of the fingers – the thumb, index finger, middle finger and the thumb side of the ring finger;
  • Depression;
  • Forgetfulness, poor memory;
  • Inability to concentrate;
  • Bradycardia (slow heart rate);

The skin and subcutaneous tissues in case of severe hypothyroidism are infiltrated with hyaluronic acid and, therefore, is swollen and has a puffy appearance – this condition is referred to as myxedema, especially apparent around the eyes, on the hands and feet. The tongue is also enlarged.  Heart rate and cardiac output decrease. Despite the reduction of a person’s appetite affected individual usually gains weight.

The gland itself may appear enlarged (presenting hypothyroid goiter), although not necessarily.

Women may present with galactorrhea (the secretion of milk) and menstrual disturbances (excessive menstrual bleeding/absence of the periods or irregular periods).

Congenital hypothyroidism

Congenital hypothyroidism affects about 1 baby in 4000 newborns. The symptoms suggestive of congenital hypothyroidism are prolonged jaundice in a newborn baby, feeding problems, failure to thrive, poor muscle tone, enlarged tongue, a hoarse cry, and umbilical hernia. Unless the diagnosis is made and the appropriate treatment is initiated early the child may present intellectual retardation as the thyroid hormones are crucial for the nervous system development. Severe hypothyroidism in infancy and childhood is called cretinism and is characterized by the mental and growth retardation.


In primary hypothyroidism, TSH levels are elevated, whereas T3 and T4 appear normal (in subclinical hypothyroidism) or decreased.

In central hypothyroidism, thyroid function tests results appear decreased.


Thyroid hormone replacement with levothyroxine (LT4) is the standard treatment for hypothyroidism.

Applicable medicines

Levothyroxine is usually prescribed at an initial dose of 50 µg per day, the dose later is adjusted in order to maintain the concentration of TSH in the blood within the reference values. A maintenance dose of levothyroxine is 100-150 µg per day. Thyroid function tests should be performed every year after the dose was established.

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