What is the most common cause of hypothyroidism (underactive thyroid)?

From the Guidelines

The most common cause of hypothyroidism is chronic autoimmune (Hashimoto) thyroiditis. This condition is characterized by the immune system attacking the thyroid gland, leading to a decrease in thyroid hormone production 1.

Key Points

• Risk factors for hypothyroidism include female sex, advancing age, white race, type 1 diabetes, Down syndrome, family history of thyroid disease, goiter, previous hyperthyroidism, and external-beam radiation in the head and neck area 1.
• The serum TSH test is the primary screening test for thyroid dysfunction, with multiple tests done over a 3- to 6-month interval to confirm or rule out abnormal findings 1.
• The principal treatment for hypothyroidism is oral T4 monotherapy (levothyroxine sodium) 1.

Diagnosis and Treatment

Diagnosis of hypothyroidism involves blood tests measuring thyroid-stimulating hormone (TSH) and free T4 levels, with elevated TSH and low T4 confirming hypothyroidism. Treatment consists of synthetic thyroid hormone replacement, typically levothyroxine, usually taken as a daily oral medication on an empty stomach 1. Regular blood tests are necessary to monitor hormone levels and adjust dosage. Medication is generally lifelong, as the condition is chronic. Other causes of hypothyroidism, such as iodine deficiency, are less common in countries with iodized salt 1.

From the Research

Causes of Hypothyroidism

• The most common cause of hypothyroidism is chronic autoimmune thyroiditis, also known as Hashimoto's thyroiditis 2, 3, 4.
• Other causes of hypothyroidism include:
  • Drugs such as amiodarone, lithium, and immune checkpoint inhibitors 2.
  • Radioactive-iodine treatment 2.
  • Thyroid surgery 2.
  • Severe iodine deficiency, which was historically the most common cause 2.
• Hashimoto's thyroiditis is an autoimmune disorder that affects women 7-10 times more often than men, and is characterized by thyroid infiltration by lymphocytes and the presence of antibodies against thyroid peroxidase (TPOAbs) 5.

Prevalence of Hashimoto's Thyroiditis

• Hashimoto's thyroiditis is the leading cause of hypothyroidism in iodine-sufficient areas of the world 4.
• About 20-30% of patients suffer from Hashimoto's thyroiditis 4.
• The disease is thought to be caused by a combination of genetic susceptibility and environmental factors that lead to the loss of immunological tolerance and an autoimmune attack on the thyroid tissue 4.

Treatment of Hypothyroidism

• The treatment of hypothyroidism typically involves daily assumption of synthetic levothyroxine 4.
• The dose of levothyroxine used for treatment is based on the degree of preserved thyroid functionality and lean body mass, and usually ranges from 1.4 to 1.8 mcg/kg/day 5.
• Levothyroxine is the treatment of choice due to its well-demonstrated efficacy, safety, and ease of use 3.