The thyroid gland is central to regulating metabolism, energy production, and growth. Disorders of the thyroid can result in either hypothyroidism (underactive thyroid) or hyperthyroidism (overactive thyroid).
Hyperthyroidism, most commonly caused by Graves’ disease, toxic multinodular goiter, or thyroid adenoma, is characterized by increased thyroid hormone production, leading to weight loss, heat intolerance, tremors, tachycardia, and anxiety.
To manage this condition, antithyroid drugs (thionamides) are used. The two primary drugs are:
- Methimazole (Tapazole)
- Propylthiouracil (PTU)
These medications inhibit the synthesis of thyroid hormones and are vital in both short-term and long-term management of hyperthyroidism.
Mechanism of Action: How Antithyroid Drugs Work
Antithyroid drugs function by blocking the synthesis of thyroid hormones (T3 and T4) within the thyroid gland.
- Methimazole and PTU inhibit the enzyme thyroid peroxidase, preventing the iodination of tyrosine residues in thyroglobulin and thereby blocking thyroid hormone production.
- PTU has an additional mechanism: it inhibits the peripheral conversion of T4 into T3, making it especially useful in thyroid storm (a life-threatening hyperthyroid crisis).
These drugs do not affect preformed thyroid hormones, which is why clinical improvement may take 2–4 weeks as existing hormones are depleted.
Clinical Indications
Antithyroid drugs are prescribed in the following situations:
- Medical management of hyperthyroidism (Graves’ disease, toxic multinodular goiter, thyroid adenoma).
- Preoperative preparation before thyroidectomy, often combined with potassium iodide to reduce vascularity of the gland.
- Adjunct to radioactive iodine therapy in selected cases.
- Management of thyroid storm, where PTU is preferred due to its effect on peripheral T4 to T3 conversion.
Adverse Reactions
Both Methimazole and PTU can cause side effects ranging from mild to life-threatening.
Common Adverse Effects
- Nausea, vomiting, and GI upset
- Headache
- Skin rash and itching
- Loss of taste or hair
Severe Adverse Reactions
- Agranulocytosis (dangerous drop in white blood cells leading to infection risk)
- Exfoliative dermatitis
- Granulocytopenia
- Hepatotoxicity (especially PTU) – may cause drug-induced hepatitis
- Hypoprothrombinemia (bleeding tendency)
Patients must be educated to seek immediate medical attention for sore throat, fever, mouth ulcers, or unusual bleeding/bruising, as these may signal agranulocytosis.
Contraindications
Antithyroid drugs are contraindicated or used with caution in certain populations:
Pregnancy and Lactation:
- Methimazole is contraindicated in the first trimester due to teratogenicity.
- PTU is preferred in the first trimester because it does not cross the placenta as readily.
- Methimazole may be used later in pregnancy at the lowest possible dose.
Breastfeeding: Neither drug is recommended, as they can pass into breast milk.
Severe liver disease: PTU carries a risk of hepatotoxicity.
Patients on anticoagulants: Increased risk of bleeding.
Drug Interactions
Antithyroid drugs interact with several other medications:
- Digoxin, Beta-blockers: Decreased effectiveness due to altered thyroid hormone levels.
- Oral antidiabetics and insulin: Increased risk of hypoglycemia.
- Oral anticoagulants (Warfarin): Increased bleeding risk due to altered metabolism.
- SSRIs (Selective serotonin reuptake inhibitors): May reduce thyroid hormone effectiveness.
- Other antidepressants: Can alter effectiveness of thyroid medication.
Nursing Management
Safe use of antithyroid drugs requires careful nursing interventions:
1. Dosing & Compliance:
- Take drugs at regular intervals (e.g., every 8 hours for PTU).
- Do not increase or skip doses without provider approval.
2. Infection Monitoring:
- Educate patients to report sore throat, fever, or cough immediately.
- Monitor white blood cell counts periodically.
3. Bleeding Risk:
- Monitor for easy bruising, gum bleeding, or prolonged bleeding.
4. Weight Monitoring:
- Record weight twice a week to track response.
5. Pulse Monitoring:
- Patients may be asked to record their pulse daily, especially if tachycardia is a concern.
6. Avoid Nonprescription Drugs:
- Many over-the-counter drugs can interfere with therapy.
7. Patient Education:
- Improvement may take weeks due to preformed thyroid hormone stores.
- Therapy is often long-term and may require lifelong monitoring.
Generic vs Trade Names and Dosage
| Generic Name | Trade Name | Indication | Dose & Route |
|---|---|---|---|
| Propylthiouracil (PTU) | None | Hyperthyroidism, thyroid storm | 5–40 mg/day orally in divided doses every 8 hours |
| Methimazole (Tapazole) | Tapazole | Hyperthyroidism | Usually 5–15 mg/day orally (not listed in the infographic but clinically relevant) |
Practical Applications in Medicine
- Graves’ Disease: Antithyroid drugs are first-line in young patients or those not candidates for surgery or radioactive iodine.
- Thyroid Storm: PTU is the drug of choice, along with beta-blockers, iodine, and corticosteroids.
- Pre-Thyroidectomy: Antithyroid drugs and potassium iodide are given to stabilize thyroid hormone levels and reduce surgical complications.
Frequently Asked Questions (FAQ)
1. How long do antithyroid drugs take to work?
They usually take 2–4 weeks before symptoms improve because stored thyroid hormones must be depleted.
2. Which is safer in pregnancy – PTU or Methimazole?
- PTU is safer in the first trimester.
- Methimazole may be used in the second and third trimesters at the lowest dose.
3. What is the most dangerous side effect of PTU?
- Hepatotoxicity and agranulocytosis are the most severe risks.
4. Can antithyroid drugs cure hyperthyroidism?
They control the condition, but relapse is common. Definitive treatment often involves radioactive iodine or surgery.
5. Should patients avoid iodine while on therapy?
Yes. Foods and supplements high in iodine (e.g., seaweed) can interfere with treatment.
