Hashimoto thyroiditis

Hashimoto thyroiditis, also known as lymphocytic thyroiditis or chronic autoimmune thyroiditis, is a subtype of autoimmune thyroiditis. It is one of the most common thyroid disorders and causes of hypothyroidism ¹⁷.

Epidemiology

Hashimoto thyroiditis affects ~2% of all women (F:M = 10-15:1), most commonly in the 30-50 year range ^17,22.

Associations

• Turner syndrome

• primary thyroid lymphoma ⁶

• Hashimoto encephalopathy (rare)

• Down syndrome

• other autoimmune disorders 

  • systemic lupus erythematosus (SLE)

  • type 1 diabetes mellitus

  • rheumatoid arthritis

  • Sjogren syndrome

  • primary biliary cirrhosis

Diagnosis

The diagnosis of Hashimoto's thyroiditis is usually based on the combination of clinical features, serology results, and ultrasound findings ^17,22. However, cytology/histology remains the gold standard for diagnosis ²².

Clinical presentation

The clinical presentation is variable and some may be asymptomatic ^17,22. Patients can present with a painless goiter +/- symptoms of hypothyroidism ^17,22. There is often a gradual painless enlargement of the thyroid gland during the initial phase with atrophy and fibrosis later on in the course. Very rarely, patients can present with a painful thyroid (known as painful Hashimoto thyroiditis) ¹⁹.

A small proportion of cases (~5%) can present with hyperthyroidism (also known as Hashitoxicosis), which usually only lasts 1-2 months ¹⁶.

Pathology

There is autoimmunity to the thyroid gland which bears both humoral- and cell-mediated features. This is followed by lymphocytic infiltration of the thyroid gland with lymphoid follicles replacing thyroid follicles. This may affect the thyroid gland in either a diffuse or focal manner. Cell populations include:

• lymphocytic aggregates

• transformed follicular cells (Askanazy/oxyphilic/Hurthle cells)

Later stages show superadded fibrosis.

Markers

• TSH: high ¹⁷

• free T4: low ¹⁷

• antithyroglobulin antibodies: found in ~70% of cases ^2,17

• thyroid peroxidase antibodies (TPO): found in 90-95% of cases ^2,17

N.B. Serological markers can be variable ¹⁷.

Radiographic features

Ultrasound

Ultrasound features can be variable depending on the severity and phase of disease as well as differentiate diffuse from nodular Hashimoto thyroiditis.

Prominent reactive cervical nodes may be present, especially in level VI, but they have normal morphologic features ^ref.

Diffuse Hashimoto thyroidits

• typically an enlarged thyroid gland with a hypoechoic, diffusely heterogeneous echotexture with hypoechoic micronodules (1-7 mm) with surrounding echogenic septation ^3,4,17,22

  • micronodular ¹⁷, pseudonodular, or giraffe pattern has a positive predictive value of 95% ¹⁷

  • hypoechogenicity is associated with hypothyroidism ¹⁷

  • thyroid gland may be small/atrophic in chronic cases ^ref

• color Doppler study usually shows slight diffuse hypervascularity, but occasionally there might be marked hypervascularity similar to a thyroid inferno ^17,22

  • marked hypervascularity does not reflect thyrotoxicosis; indeed it appears to be more common in hypothyroid Hashimoto patients ¹¹

Nodular Hashimoto thyroiditis

• also known as focal Hashimoto or lymphocytic thyroiditis

• nodules are larger at 15-18 mm (6-30 mm) diameter ^17,22

• more commonly occurs in the setting of diffuse Hashimoto thyroiditis (~75%, range 55-85%) than normal thyroid parenchyma ²²

• no typical sonographic appearance ¹⁷

  • most commonly a hypoechoic nodule with a thin hypoechoic halo and no calcification ^17,22

  • nodules can also be echogenic or isoechoic ^17,22

  • may be solitary or multiple ¹⁷

  • cystic components and calcification are uncommon ^17,22

  • margins may be smooth or irregular ^17,22

  • variable vascularity ^17,22

Nuclear medicine

Radioactive iodine

• early stages: may show increased uptake ^ref

• late stages: single or multiple areas of reduced uptake (cold spots) ^ref

FDG PET

• diffuse high uptake throughout the thyroid is consistent with chronic thyroiditis (or a normal variant) ^14,15

• superimposed focal high uptake should raise concern for a thyroid nodule including the possibility of carcinoma ^ref

Treatment and prognosis

Life-long oral administration of L-thyroxine (T4) is often required ^ref.

Complications

• association with increased risk of the following thyroid cancers ²¹

  • papillary thyroid cancer (1.7x increased risk) ²¹

  • medullary thyroid cancer (2.7x increased risk) ²¹

  • thyroid lymphoma (13x increase risk) ^6,21

• myxedema coma due severe hypothyroidism (rare) ²⁰

History and etymology

It was first described in 1912 by Hakaru Hashimoto (1881-1934), a Japanese physician ⁷, while working in Germany. In his original description, he called it "struma lymphomatosa" ¹³.

Differential diagnosis

For ultrasound appearances consider:

• individual nodules in nodular Hashiomoto thyroiditis have overlapping imaging appearances with papillary thyroid carcinoma ^17,18

• subacute granulomatous (de Quervain) thyroiditis

• thyroid lymphoma (rare)

Practical points

• patients are at higher risk for papillary thyroid carcinoma, so a discrete nodule should be considered for biopsy