As the inflammatory process progressively destroys thyroid tissue, compensatory hyperplasia may be unable to produce adequate thyroid hormone, and partial or severe hypothyroidism results. Almost 100 percent of cases of primary hypothyroidism in adults are secondary to Hashimoto's thyroiditis. This association should be sought regardless of which manifestation is first recognized. Conversely, patients found to be hypothyroid should be carefully examined for goiter, and antibody titers should be measured to confirm the underlying pathological condition. Hypothyroidism without goiter in the adult is most often caused by a late stage of chronic thyroiditis, particularly the variant in which fibrosis, rather than inflammation or hyperplasia, has dominated the pathological process.
Patients usually have very high antibody titers, which establish the diagnosis.
Variants and special features of Hashimoto's thyroiditis provide interesting challenges. Some patients have high RAIU values and frank hyperthyroidism. Because patients with Graves' disease can have lymphocytic infiltration in the thyroid and high antibody titers, this syndrome is a bridge between the two disorders and is sometimes referred to as hashitoxicosis. The disease overlaps in occurrence with other autoimmune diseases, including pernicious anemia, myasthenia gravis, Addison's disease, and premature ovarian failure, as well as with diabetes mellitus. The existence of any of these conditions except diabetes should enhance suspicion of thyroid disease.