Iodine Preparations

Iodine preparations are used in three ways: physiological supplementation to prevent deficiency, pharmacological iodide to block thyroid hormone release (thyroid storm, preparation for thyroidectomy), and thyroid blockade after exposure to radioactive iodine. Dose and indication differ completely between these uses.

Mechanism. The thyroid needs about 150 micrograms of iodine daily for hormone synthesis. In pharmacological doses, iodide inhibits organification and hormone release (the Wolff-Chaikoff effect) and reduces gland vascularity. The effect appears within 24 to 48 hours and wanes after 10 to 14 days as the gland escapes. After a nuclear release, stable iodide saturates the gland and blocks uptake of radioactive iodine.

Pharmacokinetics. Rapidly absorbed, concentrated by the thyroid and salivary glands through the sodium-iodide symporter, and excreted by the kidney.

Potassium iodide tablets. 65 mg and 130 mg, used for thyroid blockade.

Saturated potassium iodide solution. About 50 mg of potassium iodide in each drop (1 g/mL).

Lugol solution. 5% iodine with 10% potassium iodide, about 6 mg of iodine in each drop (about 126 mg/mL).

Supplementation forms. Iodised salt, and iodised oil given orally or by injection (annual dosing) in severe deficiency regions.

• Thyroid storm: saturated potassium iodide solution 5 drops (about 250 mg) every 6 hours, or Lugol solution 8 drops every 6 to 8 hours, started at least 1 hour after the first thionamide dose so that iodine is not used for new hormone synthesis.
• Preparation for thyroidectomy in Graves disease: Lugol solution 0.1 to 0.3 mL three times daily, or potassium iodide 50 to 100 mg three times daily, for 7 to 10 days before surgery, after euthyroidism is reached with a thionamide.
• Nuclear emergency blockade: a single dose taken as soon as possible, ideally from 24 hours before to 2 hours after exposure. Adults and adolescents 130 mg, children 3 to 12 years 65 mg, infants 1 month to 3 years 32 mg, neonates under 1 month 16 mg, pregnant women 130 mg. Adults over 40 benefit little unless exposure is very high.
• Deficiency prevention: daily intake of 150 micrograms for adults, 250 micrograms in pregnancy and lactation, and 90 to 120 micrograms in children.

Special groups. Pregnancy: pharmacological doses cross the placenta and cause foetal goitre and hypothyroidism, so they are limited to storm or short pre-operative courses. Breastfeeding: iodide concentrates in milk, so avoid prolonged doses. Children: dose by age for blockade. Older adults: toxic nodular goitre may worsen on iodide. Regional: iodine deficiency is common in highland and inland areas; introducing iodised salt can provoke iodine-induced hyperthyroidism in people with long-standing nodular goitre.

Adverse effects. Metallic taste, salivation, rash, mucosal irritation (iodism), hypersensitivity, thyroid dysfunction (iodine-induced hyperthyroidism in nodular goitre, hypothyroidism in autoimmune thyroiditis or after prolonged use) and hyperkalaemia with large potassium salt doses.

Contraindications. Iodine hypersensitivity (not equal to shellfish allergy), and pharmacological doses in toxic nodular disease without thionamide cover.

Interactions. Amiodarone is an iodine load. Lithium and iodide additively cause hypothyroidism. Potassium-sparing drugs add to potassium load.

Monitoring. Thyroid function at 4 to 6 weeks when used for more than 2 weeks; potassium in renal impairment; urinary iodine in surveys.

Referral and escalation. Thyroid storm, suspected iodine-induced thyrotoxicosis, or planned surgery.