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 at least 2~2% of all women (F:M = 15:1), most commonly in the 30-50 year range ¹⁷.

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

Clinical presentation

The clinical presentation is variable and some may be asymptomatic ¹⁷. Patients can present with a painless goitre +/- hypothyroidism ¹⁷. 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 Hashiomoto 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 differenitate 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 a enlarged thyroid gland with a hypoechoic, diffusely heterogeneousechotexture with hypoechoic micronodules (1-7 mm) with surrounding echogenic septations ^173,4,17,22

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

  • hypoechogenicity is associated with hypothyroidism ¹⁷

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

• hypoechoic micronodules (1-7 mm) with surrounding echogenic septations has a high positive predictive value of 95% ^3,41,7; this appearance may be described as a micronodular ¹⁷, pseudonodular, or giraffe pattern

• colour Doppler study usually shows slightlyslight 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 lymphocytic thyroiditis

• nodules are larger at ~15 mm (6-30 mm) diameter ¹⁷

• no typical sonographic appearance ¹⁷

  • when co-existing with diffuse Hashimoto thyroiditis, Hashimoto nodules are more likely to be solitary, echogenic, thin hypoechoic halo, with no calcifications ¹⁷

  • when occuring in normal thyroid parenchyma, Hashiomoto nodules are more likely to be multiple, hypoechoic, have cystic elements and peripheral eggshell calcificaiton ¹⁷

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

• myxoedema comaassociation with increased risk of the following thyroid cancers ^ref21

  • papillary thyroid cancer (1.7x increased risk) ²¹

  • medullary thyroid cancer (2.7x increased risk) ²¹

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

• myxoedema 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:

• thyroid lymphoma

• nodular Hashiomoto thyroiditis has 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