Hashimoto Thyroiditis 

Hashimoto thyroiditis (or Hashimoto’s thyroiditis) is characterized by the destruction of thyroid cells by various cell- and antibody-mediated immune processes. It is the most common cause of hypothyroidism in the United States after age 6 years. Hashimoto thyroiditis is part of the spectrum of autoimmune thyroid diseases (AITDs). (See Etiology, Presentation, and Workup.)

By strict criteria, Hashimoto thyroiditis is a histologic diagnosis first described by Hakaru Hashimoto, a Japanese surgeon working in Berlin, Germany. His report, published in 1912, was based on the examination of 4 postoperative cases. He is also credited with introducing the term struma lymphomatosa in reference to the syndrome.

Other variants of AITD include the following conditions:

• Atrophic thyroiditis
• Juvenile thyroiditis^[1]
• Postpartum thyroiditis
• Silent thyroiditis
• Focal thyroiditis

The initiating process in Hashimoto thyroiditis is not well understood.^{[2, 3, 4, 5]} The thyroid gland is typically goitrous but may be atrophic or normal in size. Antibodies binding to and blocking the thyroid-stimulating hormone (TSH) receptor have also been described and may contribute to further impairment in thyroid function. The result is inadequate thyroid hormone production and secretion, although initially, preformed thyroxine (T4) and triiodothyronine (T3) may "leak" into the circulation from damaged cells.

Patients with Hashimoto thyroiditis have antibodies to various thyroid antigens, the most frequently detected of which include anti-thyroid peroxidase (anti-TPO), antithyroglobulin (anti-Tg), and to a lesser extent, TSH receptor-blocking antibodies. Nevertheless, a small percentage of patients with Hashimoto thyroiditis (approximately 10-15%) may be antibody negative.

Other antithyroid antibodies found in AITD (including Hashimoto thyroiditis) include thyroid-stimulating antibody and cytotoxic antibody.

Hashimoto thyroiditis has a markedly higher clustering of other autoimmunity diseases, including pernicious anemia, adrenal insufficiency, and type 1 diabetes mellitus.^{[6, 7]}

In a study of 830 patients with Hashimoto thyroiditis, Tagami et al reported slight, but significant, increases in TSH serum levels and decreases in free T4 serum levels, with increasing patient age. In addition, TSH levels were positively correlated with levels of total cholesterol, triglycerides, low-density lipoprotein (LDL), high-density lipoprotein (HDL), and non-HDL, as well as with the ratio of LDL to HDL. Free T4 levels, on the other hand, were negatively correlated with these lipid parameters.^[8]

Occurrence in the United States

Hashimoto thyroiditis is the most common cause of hypothyroidism in the United States after age 6 years, with the incidence estimated to be 1.3% in a series of 5000 children aged 11-18 years. In adults, the incidence is estimated to be 3.5 per 1000 per year in women and 0.8 per 1000 per year in men. Incidence may be as high as 6% in the Appalachian region.

In the Colorado Thyroid Disease Prevalence Study, involving 25,862 adults, the prevalence of elevated TSH in symptomatic and asymptomatic adults was 9.5%, with a greater percentage of those involved being women. The prevalence of hypothyroidism and of thyroid disease in general increases with age.

International occurrence

Worldwide, the most common cause of hypothyroidism is iodine deficiency. However, Hashimoto thyroiditis remains the most common cause of spontaneous hypothyroidism in areas of adequate iodine intake. The annual incidence of Hashimoto thyroiditis worldwide is estimated to be 0.3-1.5 cases per 1000 persons.^{[9, 10]}

Sex- and age-related demographics

The incidence of Hashimoto thyroiditis is estimated to be 10-15 times higher in females. The most commonly affected age range in Hashimoto thyroiditis is 30-50 years, with the peak incidence in men occurring 10-15 years later. The overall incidence of hypothyroidism increases with age in men and women.

With early diagnosis, timely institution of levothyroxine replacement therapy, informed patient follow-up care, and attention to other attendant complications, the prognosis in Hashimoto thyroiditis is excellent, with patients leading a normal life. Untreated myxedema coma has a poor prognosis and a high mortality rate.

Morbidity related to Hashimoto thyroiditis typically results from failure to make the diagnosis of hypothyroidism or to institute l-thyroxine replacement therapy in adequate doses, or from failure on the part of the patient to take the replacement medication.

The increased prevalence of lipid disorders in association with untreated hypothyroidism has the potential to increase morbidity from coronary artery disease.

The risk for papillary thyroid carcinoma is increased in patients with Hashimoto thyroiditis.^[11] These cancers are not clearly more aggressive than other papillary thyroid carcinomas.

Therapeutic complications

Complications of overreplacement with levothyroxine sodium Include the following:

• Accelerated bone loss
• Reduction in bone mineral density
• Osteoporosis
• Increased heart rate
• Increased cardiac wall thickness
• Increased contractility

The last three problems above increase the risk of cardiac arrhythmias (especially atrial fibrillation), particularly in the elderly population.

Patients should know that thyroid replacement therapy in Hashimoto thyroiditis is, except in very rare cases, lifelong. Patients must be informed about the importance of compliance with their replacement therapy and must be instructed to report any symptoms suggestive of hyperthyroidism caused by overreplacement.

Patients must be instructed to separate—by at least 4 hours—ingestion of levothyroxine from ingestion of cholestyramine, ferrous sulfate, sucralfate, calcium carbonate, aluminum hydroxide (and other antacids), and iron-containing multivitamins, all of which impair the absorption of levothyroxine.

For patient education information, see the Thyroid and Metabolism Center, as well as Thyroid Problems.