Most Common Cause of Hypothyroidism
In areas with adequate dietary iodine supply (developed world), Hashimoto thyroiditis (chronic autoimmune thyroiditis) is the most common cause of hypothyroidism, accounting for up to 85% of cases. 1, 2, 3
Global Context
Worldwide, iodine deficiency remains the most common cause of all thyroid disorders, including hypothyroidism. 1, 2, 4
However, in industrialized nations where dietary iodine is adequate, the etiology shifts dramatically to autoimmune disease. 1
Primary Causes in Developed Countries
Hashimoto Thyroiditis (Chronic Autoimmune Thyroiditis)
This autoimmune condition is responsible for 85% of primary hypothyroidism cases in iodine-sufficient areas. 3, 4
The disease involves excessively stimulated CD4+ T cells attacking thyroid tissue, leading to progressive gland failure. 5
Genetic susceptibility, environmental factors, and gender differences (more common in women) all contribute to its development. 5
Diagnosis is confirmed by elevated anti-thyroid peroxidase (anti-TPO) antibodies, which identify the autoimmune etiology. 6, 5
Other Common Causes
Iatrogenic causes include thyroidectomy, radioiodine therapy, and external neck irradiation. 1
Drug-induced hypothyroidism occurs with medications such as amiodarone, lithium, and immune checkpoint inhibitors. 1, 3, 4
Iodine excess or deficiency can both trigger thyroid dysfunction. 1
Epidemiology
Hypothyroidism affects 0.3-12% of the population worldwide, with higher prevalence in women and older adults. 3
Clinical hypothyroidism occurs in 0.5-1.9% of women and <1% of men. 7
Subclinical hypothyroidism is more common, affecting 3-13.6% of women and 0.7-5.7% of men. 7
Over 99% of affected patients have primary hypothyroidism (thyroid gland failure) rather than secondary hypothyroidism (pituitary/hypothalamic dysfunction). 2
Clinical Significance
The distinction between iodine-deficient and iodine-sufficient regions is critical for understanding local epidemiology and public health interventions. 2, 4
In regions with salt iodination programs, the incidence of overt hypothyroidism may paradoxically increase as previously subclinical cases become manifest. 7
Untreated hypothyroidism leads to serious complications including cardiovascular disease, heart failure, infertility, and in severe cases, myxedema coma with up to 30% mortality. 3, 7
Important Caveats
The prevalence of Hashimoto thyroiditis as the leading cause applies specifically to developed countries with adequate iodine intake—this cannot be extrapolated to iodine-deficient regions. 1, 2, 4
Salt iodination programs, while preventing iodine deficiency disorders, may unmask or precipitate autoimmune thyroid disease in genetically susceptible individuals. 7
Patients with one autoimmune condition (like Hashimoto thyroiditis) are at increased risk for other autoimmune diseases, including type 1 diabetes and adrenal insufficiency. 1, 5