Transient periods of thyrotoxicosis (over-activity of the thyroid) sometimes occur, and rarely the illness may progress to full hyperthyroid Graves disease with active orbitopathy (bulging, inflamed eyes). Rare cases of fibrous autoimmune thyroiditis present with severe shortness of breath and difficulty swallowing, resembling aggressive thyroid tumors, but such symptoms always improve with surgery or corticosteroid therapy. Although primary thyroid B-cell lymphoma affects fewer than one in 1000 persons, it is more likely to affect those with long-standing autoimmune thyroiditis, as there is a 67 to 80 fold increased risk of developing primary thyroid lymphoma in patients of Hashimotos thyroiditis. Epidemiology
Hashimotos thyroiditis disorder is thought to be the most common cause of primary hypothyroidism in North America. Within person, place, and time descriptive trends of epidemiology, it becomes more clear on how Hashimotos thyroiditis develops in and impacts differing populations. Personal characteristic trends
Overall, Hashimotos thyroiditis affects up to 2% of the general population. About 5% of Caucasians will develop Hashimotos at some point in their lives. In the US, the African-American population experiences it less commonly but has greater associated mortality. It is also less frequent in  Asian populations. About 1.0 to 1.5 in 1000 people have this disease at any time. It occurs between 8 and 15 times more often in women than in men. Some research suggests a connection to the role of the placenta as an explanation for the sex difference.
A hibernoma is a benign neoplasm of vestigial brown fat. The term was originally used by the French anatomist Louis Gery in 1914. Signs and symptoms
Patients present with a slow-growing, painless, solitary mass, usually of the subcutaneous tissues. It is much less frequently noted in the intramuscular tissue. It is not uncommon for symptoms to be present for years. Benign neoplasm with brown fat is noted. Diagnosis
Imaging findings
In general, imaging studies show a well-defined, heterogeneous mass, usually showing a mass which is hypointense to subcutaneous fat on magnetic resonance T1-weight images. Serpentine, thin, low signal bands (septations or vessels) are often seen throughout the tumor. Pathology findings
From a macroscopic perspective, there is a well-defined, encapsulated or circumscribed mass, showing a soft, yellow tan to deep brown mass. The size ranges from 1 to 27 cm, although the mean is about 10 cm. The tumors histologically resemble brown fat. There are four histologic types recognized, but one is the most frequently seen (typical). There is a background of rich vascularity. Lobular type: Variable degrees of differentiation of uniform, round to oval cells with granular eosinophilic cells with prominent borders, alternating with coarsely multivacuolated fat cells (pale cells). There are usually small centrally placed nuclei without pleomorphism. The cells have large cytoplasmic lipid droplets interspersed throughout.