Autoimmune Thyroid Disease — Causes, Symptoms & Treatment Guide — Symptoms, Causes & Treatment | MyMedicPlus

Autoimmune thyroid disease (AITD) encompasses two primary conditions: Hashimoto's thyroiditis (chronic autoimmune thyroiditis — the most common cause of hypothyroidism in iodine-sufficient countries) and Graves' disease (the most common cause of hyperthyroidism). Both involve the immune system generating autoantibodies against thyroid antigens — thyroid peroxidase (TPO), thyroglobulin (Tg), and thyroid-stimulating hormone receptor (TSHR). Hashimoto's leads to progressive thyroid destruction and underactivity; Graves' leads to thyroid stimulation and overactivity. AITD affects approximately 5% of the population and is 7-10 times more common in women. It often clusters with other autoimmune diseases including type 1 diabetes, rheumatoid arthritis, coeliac disease, Addison's disease, and pernicious anaemia. The autoimmune process in AITD is complex and involves both humoral and cellular immune responses; understanding these mechanisms has led to increasingly targeted treatment options and consistently improved long-term outcomes for patients managing thyroid autoimmunity and its broad range of systemic comorbidities.

Autoimmune thyroid disease results from a combination of genetic susceptibility and environmental triggers. Genetic factors: HLA-DR3 and HLA-DR4 alleles are associated with Graves' disease; HLA-DR3 and HLA-DR5 with Hashimoto's. Non-HLA genes including PTPN22, CTLA4, and CD25 contribute to immune dysregulation. Environmental triggers include: iodine excess (may trigger Hashimoto's in susceptible individuals), selenium deficiency (selenium is essential for thyroid enzyme function), viral infections (EBV, enterovirus have been implicated), postpartum immune rebound (postpartum thyroiditis affects 5-7% of women in the first year after delivery), smoking (protective for Hashimoto's but increases Graves' orbitopathy severity), stress, and immune reconstitution after anti-cancer checkpoint inhibitor therapy. Female sex hormones influence immune function, explaining the female predominance.

Hashimoto's thyroiditis (hypothyroidism): fatigue and low energy, weight gain, cold intolerance, dry skin and hair, hair loss (especially outer third of eyebrows), constipation, bradycardia, depression and cognitive slowing ('brain fog'), menorrhagia, myalgia, and goitre (painless thyroid enlargement in 75%). Symptoms develop insidiously over months to years. Graves' disease (hyperthyroidism): weight loss despite increased appetite, heat intolerance and excessive sweating, palpitations and tachycardia (may cause atrial fibrillation), tremor, anxiety and irritability, diarrhoea, menstrual irregularity, and diffuse goitre with bruit. Graves'-specific features: exophthalmos (proptosis — bulging eyes from orbital soft tissue inflammation in 25-50%), pretibial myxoedema (non-pitting skin thickening on shins), and thyroid acropachy (clubbing). Graves' ophthalmopathy can cause diplopia, corneal exposure, and threatened vision.

Thyroid function tests (TFTs) are the cornerstone: TSH (thyroid-stimulating hormone) is the most sensitive screening test — elevated TSH indicates hypothyroidism; suppressed TSH indicates hyperthyroidism; free T4 and free T3 confirm and quantify severity. Thyroid autoantibodies: anti-TPO antibodies (positive in 95% of Hashimoto's, 70% of Graves'), anti-thyroglobulin antibodies (positive in 60-80% of Hashimoto's), and TSH receptor antibodies (TRAb/TSHR-Ab — positive in 95% of Graves', distinguishes Graves' from other causes of hyperthyroidism). Thyroid ultrasound: Hashimoto's shows heterogeneous hypoechoic gland; Graves' shows diffusely enlarged gland with increased vascularity on Doppler. Radioiodine uptake scan: Graves' shows diffuse increased uptake (vs toxic nodule or thyroiditis — reduced or patchy uptake). Graves' ophthalmopathy is assessed by CT or MRI of the orbits and ophthalmology review. Postpartum thyroiditis follows a pattern: initial hyperthyroid phase (4-8 weeks postpartum) followed by hypothyroid phase (8-20 weeks) — most recover to euthyroid state by 12 months.

Hashimoto's hypothyroidism: levothyroxine (L-T4) replacement — synthetic thyroxine taken daily on an empty stomach (30 minutes before food), titrated to maintain TSH within the normal range (0.4-4.0 mIU/L). Starting dose: 1.6 mcg/kg/day in young adults; 25-50 mcg/day in elderly or cardiac patients. Dose adjustments required in pregnancy (increases by 30-50%) and with drugs that affect absorption (calcium, iron, PPIs). Overt hypothyroidism requires lifelong treatment; subclinical hypothyroidism (elevated TSH, normal T4) — treatment threshold is TSH above 10 mIU/L or symptomatic. Graves' hyperthyroidism — three options: Antithyroid drugs (ATD) — carbimazole (UK) or methimazole (USA): first-line medical therapy for 12-18 months; remission in 30-50% after stopping; weekly FBC monitoring initially (risk of agranulocytosis — 0.1-0.5%). Radioiodine ablation (I-131): definitive treatment causing hypothyroidism in most patients (requiring lifelong levothyroxine); contraindicated in pregnancy and significant Graves' ophthalmopathy (may worsen). Thyroidectomy (total): provides definitive cure; preferred for large goitre, active ophthalmopathy, or when rapid control needed. Graves' ophthalmopathy: smoking cessation is critical; selenium supplementation 200mcg/day in mild-moderate active GO; IV methylprednisolone for moderate-severe active GO; teprotumumab (anti-IGF-1R) for proptosis; orbital radiotherapy or decompression surgery for severe cases.

Untreated Hashimoto's hypothyroidism leads to progressive metabolic deterioration. Severe hypothyroidism causes myxoedema — widespread non-pitting oedema (periorbital, facial, peripheral), hyponatraemia, and pericardial effusion. The most extreme complication is myxoedema coma — a life-threatening emergency presenting with altered consciousness, hypothermia, bradycardia, hypotension, and respiratory failure — with mortality of 20-50% even with treatment. Untreated hypothyroidism during pregnancy causes irreversible fetal neurological damage — maternal TSH above 4 mIU/L in the first trimester is associated with impaired child IQ and developmental delay. Chronic untreated hypothyroidism causes dyslipidaemia (elevated LDL cholesterol) increasing cardiovascular risk. Untreated Graves' hyperthyroidism causes atrial fibrillation in up to 10-15% — with risk of cardioembolic stroke — as well as high-output cardiac failure, severe osteoporosis from elevated bone turnover, and thyrotoxic periodic paralysis (hypokalemic muscle weakness). Thyroid storm — an acute exacerbation of Graves' disease triggered by surgery, infection, or radioiodine — presents with extreme hyperthermia, tachyarrhythmia, agitation, and organ failure with a mortality of 20-30% even with intensive treatment. Graves' ophthalmopathy, if untreated, can cause corneal exposure keratopathy, corneal ulceration, and optic neuropathy leading to permanent blindness.

Autoimmune thyroid disease cannot be prevented, but severity and progression can be managed. Selenium supplementation (200 mcg/day) reduces TPO antibody levels and may slow Hashimoto's progression in selenium-deficient populations. Maintain adequate iodine intake (150 mcg/day for adults, 220-290 mcg/day in pregnancy) from iodised salt and seafood — but avoid excessive supplementation. Vitamin D deficiency is common in autoimmune thyroid disease and supplementation to maintain 25-OH vitamin D above 75 nmol/L may support immune regulation. Smoking strongly worsens Graves' ophthalmopathy — smoking cessation is essential for any patient with Graves' disease. Women with known thyroid autoimmunity planning pregnancy or pregnant should have TSH monitored as hypothyroidism in early pregnancy causes irreversible fetal neurological damage.

See a GP if you have persistent unexplained fatigue, unexplained weight gain or loss, palpitations, heat or cold intolerance, or a visible neck swelling. Women with thyroid autoantibodies planning pregnancy should see an endocrinologist before conception — even subclinical hypothyroidism in early pregnancy causes fetal neurodevelopmental impairment. Seek urgent care for thyroid storm (Graves' crisis): extreme tachycardia, fever, agitation, and confusion — a life-threatening emergency requiring immediate hospital admission. Seek urgent review if you develop agranulocytosis symptoms while on antithyroid drugs — sore throat, fever, or mouth ulcers require immediate blood count testing (FBC), as agranulocytosis is life-threatening. Patients with Graves' ophthalmopathy with sudden visual changes, corneal pain, or inability to close the eye need same-day ophthalmology review.