Potassium Iodide

Potassium iodide is an iodine salt used for short-term blocking of thyroid hormone release and synthesis before thyroidectomy and in thyroid storm, for thyroid protection after a radiation release, and in some regions to correct iodine deficiency. Its effect on hyperthyroidism is brief, with escape after 10 to 14 days, so it is a bridge and never a stand-alone long-term treatment.

Classification. Inorganic iodide; thyroid blocker. Tablets 32.5 mg, 65 mg and 130 mg; saturated solution (about 1 g/mL, 50 mg iodide in each drop); strong iodine solution (Lugol's solution, 126 mg iodine per mL, about 6.3 mg in each drop).

Mechanism. Large iodine loads inhibit organification and release of thyroid hormone (Wolff-Chaikoff effect and blocked proteolysis of thyroglobulin), and lower thyroid blood flow and gland vascularity. As a radiation countermeasure, stable iodide saturates the gland and blocks uptake of radioactive iodine.

Pharmacodynamics. Onset within 24 hours; peak hormone reduction in 3 to 7 days; escape after about 10 to 14 days.

Pharmacokinetics. Well absorbed orally; taken up by the thyroid and concentrated in saliva, breast milk and gastric juice; excreted in urine. Crosses the placenta.

Preoperative preparation for thyroidectomy in Graves' disease.

• Saturated solution 50 mg (1 drop) three times daily, or strong iodine solution 0.1 to 0.3 mL (about 3 to 5 drops) three times daily, for 7 to 10 days before surgery, after a thionamide has made the patient euthyroid.

Thyroid storm (emergency, critical care).

• Saturated solution 250 mg (5 drops) orally every 6 hours, or strong iodine solution 10 drops every 8 hours, starting at least 1 hour after the first thionamide dose, so that iodine does not feed new hormone synthesis. Continue for several days while definitive control is reached.

Radiation emergency, thyroid blocking.

• Single dose of stable iodide taken before or as soon as possible after exposure, ideally within 24 hours: adults and adolescents over 12 years 130 mg; children 3 to 12 years 65 mg; children 1 month to 3 years 32.5 mg; newborns up to 1 month 16 mg. Repeat only on specialist instruction, because repeat doses risk hypothyroidism.
• Pregnant and breastfeeding women take a single dose of 130 mg; avoid repeat doses.

Iodine deficiency (where used).

• Prevention in areas of deficiency is by iodised salt rather than medicinal doses; supplement when specifically directed.

Adjustments.

• Kidney: caution in severe impairment (hyperkalaemia from the potassium load).
• Liver: no change.
• Older adults: no change.
• Weight: dose by age band in children.

Contraindications. Hypersensitivity to iodide; hyperkalaemia; iodide-induced thyroid dysfunction; dermatitis herpetiformis and hypocomplementaemic vasculitis (for radiation blocking); untreated toxic nodular goitre and Graves' disease for long-term radiation protection use (risk of iodine-induced hyperthyroidism); do not start before a thionamide in storm.

Serious adverse effects.

• Iodine-induced hyperthyroidism (Jod-Basedow) in nodular goitre, and iodine-induced hypothyroidism and goitre (Wolff-Chaikoff effect) in autoimmune thyroid disease.
• Hypersensitivity: angioedema, serum sickness, fever and rash.
• Hyperkalaemia.
• Iodism: metallic taste, burning mouth, parotid swelling, coryza, acneiform rash with prolonged high doses.
• Neonatal goitre and hypothyroidism after repeated doses in pregnancy.

Major interactions.

• Thionamides: give the iodide after the thionamide in storm.
• Lithium: added antithyroid and goitrogenic effect.
• Potassium-sparing diuretics, ACE inhibitors, angiotensin receptor blockers: hyperkalaemia.
• Amiodarone: iodine load; thyroid dysfunction.
• Radioactive iodine therapy: iodide before treatment blunts uptake; avoid for weeks.

Special groups.

• Pregnancy: avoid prolonged use; a single protective dose is acceptable in a radiation emergency, with newborn thyroid testing.
• Breastfeeding: single dose acceptable; avoid repeat doses; check the infant's thyroid.
• Children: age-banded single doses for radiation blocking.
• Older adults: caution in nodular goitre.
• Kidney impairment: caution; liver impairment: no change.
• East Africa: stock is rare outside referral centres, and Lugol's solution may be prepared by hospital pharmacy; check concentration and drop size when converting doses.

Formulation and administration. Dilute drops in a glass of water, juice or milk to reduce taste and gastric irritation, and take after meals. Tablets are swallowed whole with water or crushed for children. Store in a light-resistant container at room temperature.

Baseline. Free T4, TSH, potassium, history of iodine allergy and thyroid disease.

Monitoring.

• Free T4 and symptoms every few days during storm treatment and preoperative preparation.
• Potassium in kidney impairment and when combined with potassium-raising drugs.
• Newborn TSH after any maternal exposure in pregnancy.
• Thyroid function weeks after a radiation dose in infants and in people with thyroid disease.

Dose reduction and stopping. Stop on the day of thyroidectomy (preoperative use) or when the storm is controlled. Stop at once for hypersensitivity, iodism or hyperkalaemia.

Referral and escalation. Refer pregnant women, newborns, and anyone with thyroid disease after a radiation dose to endocrinology. Admit thyroid storm.