A thiourea antithyroid agent that inhibits the synthesis of thyroid hormone. It is used in the treatment of hyperthyroidism. Methylthiouracil is an organosulfur compound that is used in antithyroid preparation. It is a thioamide, closely related to propylthiouracil. Methylthiouracil is not used clinically in the United States, it has a similar mechanism of action and side effects to that of propylthiouracil. The drug acts to decrease the formation of stored thyroid hormone, such as thyroglobulin in the thyroid gland. The clinical effects of the drug to treat the hyperthyroid state can have a lag period of up to two weeks, depending on the stores of thyroglobulin and other factors.
Methylthiouracil appears as white crystalline powder with an odor of onions and a bitter taste. A saturated aqueous solution is neutral or slightly acidi
Mechanism of Action:
Methylthiouracil works by inhibiting the enzyme thyroid peroxidase in the thyroid gland. This enzyme is responsible for the production of thyroid hormones. By inhibiting this enzyme, Methylthiouracil reduces the synthesis of the thyroid hormones thyroxine (T4) and triiodothyronine (T3).
Methylthiouracil inhibits the synthesis of thyroid hormones (T3 and T4) by interfering with the iodination of tyrosine residues in thyroglobulin. This ultimately results in decreased thyroid hormone production.
Indications:
- Graves’ disease
- Toxic multinodular goiter
- Thyroid storm or thyrotoxic crisis (often as an adjunct therapy)
- Preparation for thyroidectomy in hyperthyroid patients
- Prior to radioactive iodine therapy
- Amiodarone-induced thyrotoxicosis
- Thyrotoxicosis factitia (secondary to ingestion of excess thyroid hormone)
- Ectopic thyroid tissue producing excess hormone
- Transient hyperthyroidism of hyperemesis gravidarum
- Subacute thyroiditis during the thyrotoxic phase
Contraindications:
- Known hypersensitivity to methylthiouracil or its components.
- Agranulocytosis induced by previous antithyroid drug therapy.
- Patients with bone marrow suppression.
- Liver disease or dysfunction.
- During lactation (because of potential excretion in breast milk).
- Children (in certain regions due to a higher risk of severe liver injury).
- After radioactive iodine therapy (until its effects are clear).
- Malignant thyroid nodules.
- History of vasculitis associated with antithyroid drug use.
- Severe dermatologic reactions to other antithyroid drugs.
Dosages:
The specific dosage depends on the clinical condition and the individual patient’s response. A typical starting dose for hyperthyroidism might be in the range of 30-40 mg daily, divided into several doses, with maintenance doses being lower. Regular monitoring of thyroid function and clinical condition is necessary, and the dosage is adjusted accordingly.
Side Effects:
- Agranulocytosis (low white blood cell count)
- Hepatitis
- Skin rash
- Urticaria (hives)
- Pruritus (itching)
- Arthralgia (joint pain)
- Myalgia (muscle pain)
- Fever
- Nausea
- Vomiting
- Abdominal pain
- Liver function abnormalities
- Jaundice (yellowing of the skin and eyes)
- Hair loss
- Taste disturbances
- Peripheral neuritis
- Dizziness
- Headache
- Edema (swelling)
- Lymphadenopathy (enlarged lymph nodes)
- Sialadenopathy (inflammation of salivary glands)
- Vasculitis
- Thrombocytopenia (low platelet count)
- Lupus-like syndrome
- Anemia
- Vertigo
- Paresthesia (tingling or numbness)
- Insomnia
- Depression
- Eosinophilia (increased eosinophils in the blood)
Drug Interactions:
Listing specific drug interactions is quite extensive, but here are some general categories and specific examples:
- Drugs that alter thyroid function or thyroid hormone replacement, like levothyroxine.
- Anticoagulants like warfarin (may enhance anticoagulant effects).
- Beta-blockers: enhanced effects.
- Digoxin: effects may be altered.
- Theophylline: clearance may be decreased in hypothyroidism.
- Drugs metabolized by liver: antithyroid drugs may affect liver enzyme activity and thus drug metabolism.
- Amiodarone: may cause or exacerbate hyper- or hypothyroidism.
- Antidiabetic agents: altered requirements.
- Lithium: may inhibit the release of thyroid hormones.
- Iodine-containing agents: may reduce the efficacy of antithyroid drugs. …and many others. Consultation with a pharmacist or use of a drug interaction checker tool is recommended when considering multiple medications.
Pregnancy and Lactation:
- Pregnancy: Methylthiouracil is categorized as a category D drug by the FDA, meaning there is evidence of human fetal risk, but the benefits might outweigh the risks in certain situations. The drug can cross the placenta and may cause hypothyroidism in the fetus.
- Lactation: Methylthiouracil is excreted in breast milk in small amounts. While some sources suggest avoiding its use during lactation, others indicate it can be used with caution. Monitoring the infant’s thyroid function is essential if the medication is used.
Note: Always consult with a healthcare professional before making decisions regarding medication use.
- Alternatives: Other antithyroid medications, such as propylthiouracil (PTU) and methimazole, can also be used to treat hyperthyroidism. The choice of drug often depends on the patient’s specific condition, the severity of the hyperthyroidism, possible side effects, and other factors.
If you or someone you know is prescribed Methylthiouracil or any other medication, it’s essential to follow the doctor’s instructions carefully, be aware of potential side effects, and report any unusual symptoms to a healthcare professional immediately.
