What anti-thyroid (antithyroid) therapy is suitable for intravenous (IV) administration?

Intravenous Antithyroid Therapy Options

No antithyroid medications are FDA-approved for intravenous administration in the United States, but IV methimazole can be compounded for emergency use when oral/rectal routes are unavailable.

Available IV Antithyroid Options by Region

United States

• No commercially available IV formulations exist 1, 2
• Compounded IV methimazole can be prepared using aseptic technique: 500 mg methimazole USP powder reconstituted with pH-neutral 0.9% sodium chloride to 50 mL final volume, filtered through 0.22-micron filter, administered as slow IV push over 2 minutes 2
• This preparation has successfully rendered hyperthyroid patients euthyroid when oral/rectal routes were contraindicated 2

Europe and Japan

• IV methimazole is commercially available and represents the standard parenteral formulation 1

Clinical Scenarios Requiring IV Antithyroid Therapy

IV administration becomes necessary when patients cannot tolerate oral or rectal medications 1, 2:

• Emergent gastrointestinal surgery requiring NPO status 2
• Bowel ileus or obstruction 2
• Severe intractable vomiting and diarrhea 2
• Duodenal perforation or gastrointestinal bleeding requiring bowel rest 1, 2
• Thyroid storm with inability to take oral medications 1, 3

Alternative Routes When IV Unavailable

Rectal Administration

• Propylthiouracil (PTU) enema preparations have been used successfully when IV formulations unavailable 1
• Methimazole suppositories represent another rectal option 1
• Rectal PTU was administered for five doses in documented thyroid storm case, though hepatocellular injury developed requiring transition to oral methimazole 1

Treatment Protocol for Thyroid Storm

When IV antithyroid therapy is needed for thyroid storm, the complete regimen includes 3, 4:

• Thionamides (via IV if compounded, or rectal if IV unavailable) to block hormone synthesis 1, 3
• Iodine solutions (iopanoic acid or ipodate) to block thyroid secretion, administered 1 hour after thionamides 3, 4
• Beta-blockers to reduce hypermetabolic state and control tachycardia 3, 4
• IV glucocorticoids to reduce peripheral T4 to T3 conversion 3, 4
• Plasmapheresis or thyroidectomy reserved for extreme refractory cases 1, 4

Critical Safety Considerations

Hepatotoxicity Risk

• PTU carries significant hepatocellular injury risk, as demonstrated in the case requiring discontinuation after rectal administration 1
• Methimazole is generally preferred when available due to lower hepatotoxicity profile 5

Major Adverse Effects of Thionamides

• Agranulocytosis occurs in 0.1-0.5% of patients, representing the most dangerous complication 5
• Granulocyte colony-stimulating factor effectively treats this life-threatening condition 5
• Minor adverse effects (rash, itching, mild leukopenia) occur in 3-5% of patients 5
• Aplastic anemia, thrombocytopenia, lupus-like syndrome, and vasculitis are exceedingly rare 5

Common Pitfalls to Avoid

• Never administer iodine before thionamides, as iodine can worsen thyrotoxicosis by providing substrate for additional hormone synthesis 3, 4
• Do not assume commercial IV formulations are available in the US—preparation requires compounding pharmacy capabilities 1, 2
• Monitor for hepatotoxicity closely with PTU, particularly with alternative routes of administration 1
• Ensure proper filtration (0.22-micron) when compounding IV methimazole to maintain sterility 2
• Administer IV methimazole slowly over 2 minutes, not as rapid bolus 2