What causes hypothyroidism (underactive thyroid) and how can it be corrected?

From the Guidelines

Hypothyroidism is primarily caused by chronic autoimmune (Hashimoto) thyroiditis and can be corrected with oral T4 monotherapy (levothyroxine sodium) 1.

Causes of Hypothyroidism

• The most common cause of hypothyroidism in the United States is chronic autoimmune (Hashimoto) thyroiditis 1
• Other 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

Correction of Hypothyroidism

• The principal treatment for hypothyroidism is oral T4 monotherapy (levothyroxine sodium) 1
• The treatment typically involves lifelong supplementation with levothyroxine, with the goal of achieving a target serum thyrotropin (TSH) level between 0.5-4.5 mU/L
• Monitoring of TSH levels is necessary to adjust the levothyroxine dose as needed, typically every 6-12 months 1

From the FDA Drug Label

The general aim of therapy is to normalize the serum TSH level TSH may not normalize in some patients due to in utero hypothyroidism causing a resetting of pituitary-thyroid feedback.

The cause of hypothyroidism is not directly stated in the provided drug labels. However, hypothyroidism can be corrected by normalizing the serum TSH level with levothyroxine sodium therapy. The dosage of levothyroxine sodium should be carefully titrated and monitored to avoid overtreatment or undertreatment, which may have negative effects on growth and development, cardiovascular function, and other bodily functions 2. Some factors that may affect levothyroxine sodium therapy include:

• Drug interactions: Certain medications, such as beta-adrenergic antagonists, glucocorticoids, and amiodarone, may decrease the conversion of T4 to T3 or affect thyroid hormone levels 2.
• Food interactions: Consumption of certain foods, such as soybean flour, cottonseed meal, walnuts, and dietary fiber, may affect levothyroxine sodium absorption 2.

From the Research

Causes of Hypothyroidism

• Hypothyroidism is caused by a variety of factors, including autoimmune diseases, radiation therapy, thyroid gland removal surgeries, and certain medications 3, 4, 5, 6, 7
• Chronic autoimmune thyroiditis (Hashimoto's thyroiditis) is the most common cause of hypothyroidism 4, 6
• Severe iodine deficiency is also a cause of hypothyroidism, although it is more common in areas with low iodine intake 4, 6
• Other causes of hypothyroidism include drugs such as amiodarone, lithium, and immune checkpoint inhibitors, as well as thyroid surgery 4

Symptoms of Hypothyroidism

• Common symptoms of hypothyroidism include fatigue, weight gain, increased sensitivity to cold (cold intolerance), irregular bowel movements (constipation), and dry skin (xeroderma) 3, 4, 5, 6
• These symptoms are often non-specific and can overlap with other conditions, making diagnosis more challenging 3, 4
• Hypothyroidism can also lead to more severe complications if left untreated, such as mental retardation, delayed milestones, heart failure, infertility, and myxedema coma 3, 6

Diagnosis and Treatment of Hypothyroidism

• Diagnosis of hypothyroidism is based on laboratory tests that measure the levels of thyroid hormones (T3 and T4) in the blood 3, 5, 6
• Treatment typically involves lifelong hormone replacement therapy with synthetic thyroid hormone replacement medication, such as levothyroxine 3, 4, 5, 6, 7
• Lifestyle modifications, such as eating healthy food and exercising regularly, can help manage symptoms and improve quality of life 3
• The decision to treat subclinical hypothyroidism should be based on patient characteristics and shared decision-making discussions 5
• Special consideration should be taken in treating patients with high-risk conditions, including heart disease, pregnancy, and myxedema coma, and in patients requiring high-dose levothyroxine 5, 6