Hypothyroidism Causes and Risk Factors in Adult Women

Primary Causes in Iodine-Sufficient Regions

The American College of Physicians and American Academy of Family Physicians recommend recognizing that chronic autoimmune thyroiditis (Hashimoto's disease) is the most common cause of hypothyroidism in adult women in the United States and other iodine-sufficient areas, accounting for approximately 85-90% of cases 1, 2
Radioactive iodine therapy for hyperthyroidism or thyroid cancer leads to permanent hypothyroidism, as stated by the American College of Physicians and American Academy of Family Physicians 1, 2, 3
Thyroidectomy (surgical removal of thyroid tissue) results in hypothyroidism requiring lifelong replacement, according to the American Academy of Family Physicians 2, 3
External-beam radiation to the head and neck area damages thyroid tissue, as reported by the American College of Physicians 1
Ablation therapy for previous hyperthyroidism commonly causes iatrogenic thyroid dysfunction, as stated by the American College of Physicians 1

Amiodarone (iodine-containing cardiac medication) can induce thyroid dysfunction, according to the American College of Physicians 1
Immune checkpoint inhibitors (anti-PD-1/PD-L1 therapy) cause thyroid dysfunction in 5-10% of patients, with combination immunotherapy increasing risk to 20%, as reported by the American College of Medical Genetics and Genomics 4

Iodine deficiency remains the most common cause of hypothyroidism worldwide, particularly in developing nations, as stated by the American Academy of Family Physicians 2, 3
Maternal hypothyroidism from iodine deficiency increases risk of congenital cretinism with growth failure, mental retardation, and neuropsychologic defects, according to the American Academy of Family Physicians 2

Female sex is the strongest risk factor, with hypothyroidism being significantly more common in women than men, as reported by the American College of Physicians 1
Advancing age increases risk, with prevalence rising substantially in elderly populations, according to the American College of Physicians 1
Family history of thyroid disease confers increased risk, as stated by the American College of Physicians 1
Type 1 diabetes increases risk of autoimmune thyroid disease, according to the American College of Physicians 1

Goiter may be associated with hypothyroidism development, as reported by the American College of Physicians 1
Down syndrome increases risk of thyroid dysfunction, according to the American College of Physicians 1

In regions with adequate dietary iodine, Hashimoto thyroiditis (chronic lymphocytic thyroiditis) accounts for up to 85‑90 % of hypothyroidism cases, making it the predominant cause in adult populations. The American College of Radiology highlights this prevalence in its 2019 review. 5

The American College of Radiology states that imaging modalities (ultrasound, CT, MRI, nuclear medicine) have no role in determining the underlying cause of hypothyroidism because all etiologies—including autoimmune thyroiditis, iodine deficiency, post‑ablation status, and medication‑induced disease—show uniformly decreased radioiodine uptake and cannot be distinguished by thyroid morphology. Diagnosis therefore relies on clinical history and laboratory assessment (TSH and free T4). 5