Thyrotoxicosis

The Concept

Thyrotoxicosis is the clinical state produced by an excess of circulating thyroid hormone, and its features flow directly from the fact that thyroid hormone drives the body's metabolic rate and amplifies sympathetic activity. The commonest cause is Graves' disease, an autoimmune condition in which TSH-receptor stimulating antibodies continuously activate the gland — explaining both the diffuse overactivity and the unique extrathyroidal features.

Causes

The main causes are Graves' disease (diffuse, autoimmune — commonest), toxic multinodular goitre (autonomous nodules in an older patient), toxic adenoma (a single hot nodule), and thyroiditis (a transient release of stored hormone). Recognising the cause matters because it determines treatment.

Clinical Features

The hypermetabolic, sympatho-active state produces a characteristic picture: weight loss despite a good appetite, heat intolerance and sweating, palpitations with tachycardia or atrial fibrillation, a fine tremor, anxiety and irritability, diarrhoea, oligomenorrhoea, warm moist skin, and proximal muscle weakness. A goitre is usually present.

Features Specific to Graves' Disease

Because Graves' is autoimmune, it has extrathyroidal signs not seen in other causes, which are diagnostic:

• eye disease (Graves' ophthalmopathy) — exophthalmos/proptosis, lid retraction and lid lag, and, in severe cases, ophthalmoplegia and sight-threatening optic-nerve compression
• pretibial myxoedema (thickened skin over the shins)
• thyroid acropachy (clubbing-like changes)
• a diffuse goitre with an audible bruit (from its vascularity).

Investigations

Confirm with thyroid function tests — a suppressed TSH with raised free T3/T4. TSH-receptor antibodies confirm Graves'. A radioiodine uptake scan distinguishes the causes: diffusely increased uptake in Graves', patchy uptake in toxic MNG, and a single hot area in a toxic adenoma.

Management — Three Modalities

• Antithyroid drugs — carbimazole (or propylthiouracil) block hormone synthesis and render the patient euthyroid, given as a titrated dose or 'block-and-replace'; a 12–18 month course achieves lasting remission in about half of Graves' patients. A beta-blocker (propranolol) rapidly controls the sympathetic symptoms. Patients must be warned that a sore throat may signal agranulocytosis and to have a blood count checked.
• Radioiodine (I-131) — a definitive outpatient treatment that ablates thyroid tissue; it commonly results in hypothyroidism, and is avoided in pregnancy and active eye disease.
• Surgery (thyroidectomy) — for a large or compressive goitre, suspected malignancy, failed drug therapy, or patient choice. The gland must be rendered euthyroid before operation and given Lugol's iodine pre-operatively to reduce its vascularity.

Choosing Between the Three Treatments

• Each modality has trade-offs that guide the choice.
• Antithyroid drugs are non-invasive and can achieve lasting remission in Graves' but require prolonged treatment and carry a relapse rate and the risk of agranulocytosis.
• Radioiodine is simple and definitive but usually leads to lifelong hypothyroidism and is unsuitable in pregnancy or active eye disease.
• Surgery gives rapid, definitive control and is preferred for large/compressive goitres, suspected malignancy, or when the other options fail or are contraindicated, at the cost of the operative risks. The decision is individualised to the cause, goitre size, eye disease, pregnancy plans and patient preference.

Graves' Ophthalmopathy & Pregnancy

Graves' eye disease is autoimmune and runs a course partly independent of the thyroid; it is worsened by smoking and by radioiodine, and severe, sight-threatening disease needs high-dose steroids, orbital radiotherapy or surgical decompression. In pregnancy, propylthiouracil is generally preferred in the first trimester and radioiodine is contraindicated — an important safety point.

Thyrotoxicosis in Special Situations

Two situations deserve emphasis. In pregnancy, thyrotoxicosis must be controlled to protect mother and fetus, but radioiodine is absolutely contraindicated (it crosses the placenta and ablates the fetal thyroid), so antithyroid drugs are used — propylthiouracil in the first trimester — at the lowest effective dose, with surgery reserved for drug failure. In the elderly, thyrotoxicosis may present atypically as 'apathetic thyrotoxicosis' — with weight loss, atrial fibrillation and lethargy rather than the classic hyperkinetic picture — so it must be actively considered as a cause of new atrial fibrillation or unexplained weight loss in older patients.

• Carbimazole to render euthyroid; propranolol for sympathetic symptoms; warn re agranulocytosis (sore throat → FBC).
• Lugol's iodine ~10 days pre-op to reduce gland vascularity.
• Radioiodine avoided in pregnancy and active Graves' eye disease.

Ophthalmopathy occurs only in Graves disease, not other causes.

• Thyrotoxicosis = excess thyroid hormone → hypermetabolic + sympathetic features; Graves' (autoimmune, TSH-receptor antibodies) is commonest.
• Causes: Graves', toxic MNG, toxic adenoma, thyroiditis.
• Features: weight loss with good appetite, heat intolerance, palpitations/AF, tremor, anxiety, diarrhoea; Graves'-specific: eye disease, pretibial myxoedema, acropachy, diffuse goitre with bruit.
• Diagnose: low TSH + high T3/T4, TSH-receptor antibodies, radioiodine uptake scan (diffuse/patchy/single-hot).
• Treat with antithyroid drugs (carbimazole) + propranolol, radioiodine, or surgery; render euthyroid + Lugol's iodine before surgery to prevent thyroid storm.