Hashimoto’s Thyroiditis and Primary Thyroid Lymphoma

Hashimoto’s thyroiditis (HT) is an autoimmune thyroid disorder discovered by Hakaru Hashimoto in 1912. The disease, which is the most common cause of hypothyroidism in adults, is characterized by a diffusely enlarged or atrophic thyroid with lymphocytic infiltration and the presence of thyroid autoantibodies. It arises from interactions between genetic, epigenetic, and environmental factors. HT has a prevalence of up to 10% in the US adult population and is more frequent among females. Primary thyroid lymphoma (PTL) is a rare cancer, accounting for less than 5% of all thyroid malignancies, and affecting mainly females. The purpose of this review is to present an update on the prevalence, histology, and pathogenesis of PTL in relation to HT.

Methods:

A systematic search of literature was conducted using the search terms Hashimoto’s thyroiditis, primary thyroid lymphoma, prevalence, histology, and pathogenesis.

Results:

A meta-analysis of 36 studies including 64,628 subjects with HT or different types of thyroid cancer showed that the relative risk of PTL in HT is 9.74. A meta-analysis of 38 studies including 1,346 subjects with PTL revealed that the prevalence of HT (presence of at least one diagnostic criterium) in PTL is 78.9%. The most common histotypes of PTL are diffuse large B-cell lymphoma (50 to 70% of cases) and mucosa-associated lymphoid tissue lymphoma (10 to 50% of cases). The prevalence of HT is significantly higher in PTL with mucosa-associated lymphoid tissue lymphoma. Long-standing chronic inflammation and antigenic stimulation of lymphocytes may contribute to the pathogenesis of PTL.

Discussion/Conclusion:

PTL is a rare cause of thyroid malignancy. HT is a major risk factor for developing PTL and according to different meta-analyses, there is a strong association between PTL and HT. The pathogenesis of PTL is complex and multifactorial. Chronic inflammation and antigenic stimulation may play an important role. A rapid growth of a neck mass with or without compressive symptoms in a subject with a history of HT should draw attention and trigger appropriate investigations for the diagnosis of PTL through imaging and biopsy. Fine-needle aspiration biopsy can be misleading and usually core biopsy is needed. PTL is often curable and has a good prognosis if diagnosed early.