Radioiodine Therapy

Radioiodine therapy is oral sodium iodide I-131, a beta-emitting radioisotope concentrated by thyroid tissue. It destroys overactive thyroid tissue in Graves disease and toxic nodular goitre, and ablates remnant tissue and metastases in differentiated thyroid cancer.

Mechanism. Thyroid follicular cells take up iodide through the sodium-iodide symporter. I-131 releases beta particles with a tissue range of 0.4 to 2 mm, causing local cell death while sparing neighbouring tissue. Physical half-life is 8 days.

Pharmacokinetics. Rapid gut absorption, thyroid uptake of 20 to 60% in hyperthyroidism, salivary and gastric uptake, and renal excretion of the rest. Effect on the gland builds over 6 to 12 weeks.

Benign thyroid disease. Activity 200 to 600 MBq as a fixed amount, or calculated to deliver a target tissue dose from gland size and uptake. Toxic nodular disease usually needs 400 to 800 MBq.

Thyroid cancer. Remnant ablation 1.1 to 3.7 GBq, and treatment of metastases 3.7 to 7.4 GBq, given with TSH stimulation (levothyroxine withdrawn 4 to 6 weeks, or recombinant TSH 0.9 mg intramuscularly on 2 consecutive days).

Versus thionamides and surgery. Definitive and non-invasive, but leads to hypothyroidism in most Graves disease. Surgery is preferred for large goitre with compression, suspected cancer, active moderate to severe orbitopathy or contraindications to radiation.

• Graves disease: cure in 80 to 90% after one dose, with hypothyroidism in 50 to 80% within a year.
• Toxic adenoma and toxic multinodular goitre: cure in 80 to 90%, with lower hypothyroidism rates.
• Before treatment: stop thionamides 3 to 7 days before; low iodine diet for 1 to 2 weeks; avoid iodinated contrast for 2 to 3 months; confirm a negative pregnancy test within 72 hours; and consider beta blockade in older or cardiac patients.
• Orbitopathy: in Graves disease with mild active orbitopathy or smoking, give prednisolone 0.3 to 0.5 mg/kg daily for 1 to 3 months to prevent worsening. Avoid in severe active orbitopathy.
• After treatment: thyroid function every 4 to 6 weeks, and start levothyroxine when free T4 falls under the reference range or TSH rises.

Special groups.

• Pregnancy: absolutely contraindicated; avoid conception for 6 to 12 months after treatment in women, and fathering for 4 to 6 months in men.
• Breastfeeding: contraindicated; stop at least 6 weeks before.
• Children: used selectively over age 5 years.
• Older adults: treat with thionamide first for safety.
• Radiation precautions: avoid close contact with children and pregnant women for 3 to 21 days depending on the activity, with hospital isolation for cancer doses.
• Regional: services are limited, so referral delay is common; use thionamides to control disease meanwhile.

Adverse effects. Radiation thyroiditis with neck pain (days 3 to 10), transient thyrotoxicosis, worsening orbitopathy (15 to 20% in smokers), sialadenitis and taste change at cancer doses, nausea, dry eyes, rare thyroid storm, and a small increase in secondary malignancy at high cumulative cancer doses.

Contraindications. Pregnancy, breastfeeding, inability to follow radiation safety instructions, and severe active orbitopathy.

Interactions. Thionamides, iodine, amiodarone, iodinated contrast and kelp reduce uptake. Levothyroxine and liothyronine suppress uptake and are withdrawn for cancer treatment.

Monitoring. Free T4 and TSH at 4 to 6 weeks, then every 4 to 6 weeks until stable, then annually for life. Thyroglobulin and neck ultrasonography after cancer treatment.

Referral and escalation. All patients are managed by a nuclear medicine or endocrine service. Urgent review for thyroid storm, worsening eyes, or neck swelling with breathing difficulty.