Hashimoto Thyroiditis
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Direct answer
Hashimoto thyroiditis is chronic autoimmune thyroiditis and the commonest cause of hypothyroidism in iodine-replete populations. Serum anti-thyroid peroxidase antibodies are usually present, and histology shows lymphocytic infiltrate with germinal centres, Hurthle cell change and fibrosis. The disease is linked with other autoimmune conditions and, rarely, with a primary thyroid lymphoma that examiners often ask about.
What you must remember
- Antibodies: anti-thyroid peroxidase is the marker of choice; anti-thyroglobulin is less specific.
- Histology: lymphocytic infiltrate with germinal centres, Hurthle cells and fibrosis.
- Early phase: transient thyrotoxicosis, sometimes called hashitoxicosis, can precede hypothyroidism.
- Associations: Turner and Down syndromes, Sjogren syndrome, type 1 diabetes, coeliac disease, pernicious anaemia and Addison disease.
- Lymphoma: primary thyroid B-cell lymphoma, DLBCL being commonest and MALT lymphoma the classic exam answer; suspect it with rapid growth, a firm goitre or compressive symptoms.
- Treatment: levothyroxine, with full replacement of about 1.6 micrograms per kg per day in healthy adults.
- Pregnancy: the dose often needs increasing, and antibody-positive women have a higher miscarriage risk.
A goitre that starts to grow
A 52-year-old woman has fatigue, cold intolerance and constipation. Her goitre is diffuse and firm, TSH is 14 mIU/L and free thyroxine is low.
- Confirm the antibodies: Anti-thyroid peroxidase antibodies are strongly positive.
- Start replacement: Levothyroxine is started at a full replacement dose, and TSH is rechecked after several weeks, with the dose adjusted to target.
- Review response and absorption: Her symptoms improve and the goitre becomes smaller. Levothyroxine absorption is also reviewed, since calcium, iron and proton pump inhibitors reduce it.
- Spot the red flag: Years later, a 60-year-old with the same background notices the goitre has grown rapidly over three months and now causes dysphagia.
- Investigate the mass: Ultrasound shows a hypoechoic mass, and fine needle aspiration is needed because rapid growth in a long-standing goitre raises the possibility of lymphoma.
- Shift the pathway: Histology shows a MALT-type lymphoma, and management shifts to haematology and oncology. The treatment for benign Hashimoto disease is not the answer for lymphoma, and examiners want this shift recognised.
Where the exam tries to catch you
Examiners return to the same few confusions about this disease.
- Trap: confusing Hashimoto with Graves disease because both are autoimmune Correct: Hashimoto has low or variable radioiodine uptake and anti-TPO positivity, while Graves has high uptake and TSH receptor antibodies.
- Trap: mistaking the transient thyrotoxic phase for Graves disease Correct: transient thyrotoxicosis, or hashitoxicosis, can precede the hypothyroidism of Hashimoto disease, and the low radioiodine uptake helps separate them.
- Trap: accepting a painful goitre as Hashimoto Correct: the goitre in Hashimoto is typically painless, firm and diffuse, so a painful goitre points elsewhere, to subacute thyroiditis for example.
- Trap: repeating hormone checks when a long-standing goitre grows fast Correct: primary thyroid lymphoma is the rare complication examiners test, and rapid growth or compressive symptoms should prompt tissue diagnosis rather than repeated hormone checks.
Frequently asked questions
Which antibody is most typical in Hashimoto thyroiditis?
Anti-thyroid peroxidase antibodies are the most typical serological marker in Hashimoto thyroiditis and are present in most patients. Anti-thyroglobulin antibodies are also common but less specific. Their presence supports the diagnosis in a patient with hypothyroidism and a goitre.
What histological features define Hashimoto thyroiditis?
Hashimoto thyroiditis is defined by a dense lymphocytic infiltrate with germinal centres, Hurthle cell change of follicular epithelium and progressive fibrosis. Follicular destruction and atrophy follow over time. These features distinguish it from other forms of thyroiditis.
Why does Hashimoto thyroiditis raise the risk of thyroid lymphoma?
Chronic antigenic stimulation and persistent B-cell activity in Hashimoto thyroiditis are thought to drive the development of MALT-type lymphoma. The risk is low in absolute terms but is clinically important. Rapid goitre growth or compressive symptoms should prompt biopsy.
Which autoimmune diseases are associated with Hashimoto thyroiditis?
Type 1 diabetes, Addison disease, pernicious anaemia, coeliac disease, vitiligo and Sjogren syndrome are associated with Hashimoto thyroiditis. Turner and Down syndromes are also linked. Screening for associated autoimmune conditions is reasonable when the thyroid diagnosis is made.
How is levothyroxine absorption affected in Hashimoto thyroiditis?
Levothyroxine absorption is reduced by calcium, iron, proton pump inhibitors and some foods, so it is usually taken on an empty stomach, separated from these agents. Changes in absorption can alter the required dose. TSH is rechecked after any change to the regimen.