THYRO-BLOCK

Clinical safety rating: caution

Comprehensive clinical and safety monograph for THYRO-BLOCK (THYRO-BLOCK).

How it works

THYRO-BLOCK (potassium iodide) inhibits thyroid hormone synthesis and release by blocking the oxidation of iodide to iodine, thereby preventing the incorporation of iodine into thyroglobulin. It also reduces the vascularity of the thyroid gland.

What the body does with it

Metabolism	Primarily excreted unchanged in urine; minimal hepatic metabolism.
Excretion	Primarily renal excretion of unchanged drug (80-90% of absorbed dose) via glomerular filtration; minimal biliary/fecal elimination (<5%).
Half-life	Terminal elimination half-life is approximately 36 hours (range 24-48 hours) in euthyroid patients; prolonged in hyperthyroid patients due to increased thyroid hormone binding and reduced clearance.
Protein binding	Approximately 0% (negligible protein binding); potassium iodide is freely soluble and unbound in plasma.
Volume of Distribution	Volume of distribution is approximately 0.23 L/kg, indicating distribution primarily in extracellular fluid; concentrates in thyroid gland (10-20 times plasma concentration).
Bioavailability	Oral: Bioavailability is >90% due to rapid and complete gastrointestinal absorption; intravenous administration yields 100% bioavailability.
Onset of Action	Oral: Onset of thyroid suppression occurs within 24-48 hours; peak effect on thyroid hormone uptake is achieved after 10-14 days of continuous therapy.
Duration of Action	Duration of thyroid suppression persists for 14-21 days after discontinuation, depending on thyroidal iodine turnover and renal function.

Classification & Brands

Dosing & administration

Oral, 120 mg every 12 hours (total daily dose 240 mg) starting 24 hours before exposure to radioactive iodine and continuing for 5–10 days as directed.

Dosage form	TABLET
Renal impairment	No specific dose adjustment provided by manufacturer; use with caution in severe renal impairment due to potential accumulation of iodine.
Liver impairment	No specific dose adjustment provided by manufacturer; use with caution in severe hepatic impairment.
Pediatric use	Oral: 1 drop (approximately 19 mg) daily for children <1 year; 1/2 tablet (60 mg) daily for children 1–3 years; 1 tablet (120 mg) daily for children 3–12 years; 1–2 tablets (120–240 mg) daily for children >12 years. Administer 24 hours before exposure and continue for 5–10 days.
Geriatric use	No specific dose adjustment recommended; use with caution due to potential age-related renal impairment and increased sensitivity to iodine effects.

Use during pregnancy

1st trimester	Consult provider
2nd trimester	Consult provider
3rd trimester	Consult provider

Clinical note

Comprehensive clinical and safety monograph for THYRO-BLOCK (THYRO-BLOCK).

Breastfeeding	Iodine is excreted into breast milk. M/P ratio not established. Potential for hypothyroidism in nursing infant; alternate drug recommended if possible.
Teratogenic Risk	Fetal goiter and hypothyroidism may occur when administered to pregnant women, especially after first trimester. Risk is dose- and duration-dependent; avoidance recommended unless clearly needed.
Fetal Monitoring

Warnings & precautions

■ FDA Black Box Warning

None

Side Effect Profile

Serious Effects

Absolute Contraindications

["Absolute: Known hypersensitivity to iodide.","Relative: Dermatitis herpetiformis, hypocomplementemic vasculitis, and nodular thyroid disease (may precipitate hyperthyroidism)."]

Clinical Precautions

Precautions

["Hypersensitivity reactions including angioedema and urticaria.","May cause thyroid dysfunction (hyper- or hypothyroidism), particularly in patients with pre-existing thyroid disease.","Use with caution in patients with renal impairment due to potential iodide accumulation.","May interfere with thyroid function tests.","Not for chronic use except in specific circumstances (e.g., preoperative preparation)."]

Clinical Tips & Counseling

Drug interactions

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THYRO-BLOCK

Clinical safety rating: caution

Comprehensive clinical and safety monograph for THYRO-BLOCK (THYRO-BLOCK).

How it works

What the body does with it

Metabolism	Primarily excreted unchanged in urine; minimal hepatic metabolism.
Excretion	Primarily renal excretion of unchanged drug (80-90% of absorbed dose) via glomerular filtration; minimal biliary/fecal elimination (<5%).
Half-life	Terminal elimination half-life is approximately 36 hours (range 24-48 hours) in euthyroid patients; prolonged in hyperthyroid patients due to increased thyroid hormone binding and reduced clearance.
Protein binding	Approximately 0% (negligible protein binding); potassium iodide is freely soluble and unbound in plasma.
Volume of Distribution	Volume of distribution is approximately 0.23 L/kg, indicating distribution primarily in extracellular fluid; concentrates in thyroid gland (10-20 times plasma concentration).
Bioavailability	Oral: Bioavailability is >90% due to rapid and complete gastrointestinal absorption; intravenous administration yields 100% bioavailability.
Onset of Action	Oral: Onset of thyroid suppression occurs within 24-48 hours; peak effect on thyroid hormone uptake is achieved after 10-14 days of continuous therapy.
Duration of Action	Duration of thyroid suppression persists for 14-21 days after discontinuation, depending on thyroidal iodine turnover and renal function.

Classification & Brands

Dosing & administration

Oral, 120 mg every 12 hours (total daily dose 240 mg) starting 24 hours before exposure to radioactive iodine and continuing for 5–10 days as directed.

Dosage form	TABLET
Renal impairment	No specific dose adjustment provided by manufacturer; use with caution in severe renal impairment due to potential accumulation of iodine.
Liver impairment	No specific dose adjustment provided by manufacturer; use with caution in severe hepatic impairment.
Pediatric use	Oral: 1 drop (approximately 19 mg) daily for children <1 year; 1/2 tablet (60 mg) daily for children 1–3 years; 1 tablet (120 mg) daily for children 3–12 years; 1–2 tablets (120–240 mg) daily for children >12 years. Administer 24 hours before exposure and continue for 5–10 days.
Geriatric use	No specific dose adjustment recommended; use with caution due to potential age-related renal impairment and increased sensitivity to iodine effects.

Use during pregnancy

1st trimester	Consult provider
2nd trimester	Consult provider
3rd trimester	Consult provider

Clinical note

Comprehensive clinical and safety monograph for THYRO-BLOCK (THYRO-BLOCK).

Breastfeeding	Iodine is excreted into breast milk. M/P ratio not established. Potential for hypothyroidism in nursing infant; alternate drug recommended if possible.
Teratogenic Risk	Fetal goiter and hypothyroidism may occur when administered to pregnant women, especially after first trimester. Risk is dose- and duration-dependent; avoidance recommended unless clearly needed.
Fetal Monitoring

Warnings & precautions

■ FDA Black Box Warning

None

Side Effect Profile

Serious Effects

Absolute Contraindications

["Absolute: Known hypersensitivity to iodide.","Relative: Dermatitis herpetiformis, hypocomplementemic vasculitis, and nodular thyroid disease (may precipitate hyperthyroidism)."]