Classify antithyroid agents in terms of their mechanism of actions.

Thyroid hormone synthesis inhibitors: These drugs inhibit the enzyme thyroid peroxidase, decreasing iodide oxidation, iodination of tyrosyl residues in thyroglobulin, and coupling of iodotyrosyl and iodothyronyl residues. 
  • EXAMPLES: thioamides, which include propylthiouracil, methimazole, and its prodrug carbimazole.
Iodide uptake inhibitors/ ionic Inhibitors: They decrease the uptake of iodide ions into follicular cells of the thyroid gland. 

  • EXAMPLES: potassium perchlorate, pertechnetates, thiocyanates, and nitrates.
Thyroid hormone release inhibitors: They inhibit the release of thyroid hormones by the thyroid gland.
  • EXAMPLES: The most studied drug in this class is lithium.
Thyroid hormone receptor antagonists: These drugs block the action of thyroid hormones at the cellular level. They do this by binding to thyroid hormone receptors and preventing thyroid hormones from exerting their effects.
  • Examples include: Tapazole, Methimazole
Destroy Thyroid Tissue: 
EXAMPLES: Radioactive Iodine (I-129, 123, 125)

Explain the mechanism of action of methimazole to treat Graves’ disease.                                                         


Graves’ disease is an autoimmune disorder that results in the overproduction of thyroid hormones, a condition known as hyperthyroidism. 
Treatment for Graves’ disease may include medications to reduce the production of thyroid hormone, radioiodine therapy, or surgery to remove the thyroid. 

Methimazole is a thionamide antithyroid agent that inhibits the synthesis of thyroid hormones. 
MODE of ACTION:
  1. It is actively transported to the Thyroid Gland
  2. Blocks the Action of Thyroid Peroxidase enzyme by coupling with the oxidized state of Heme group.
  3. this inhibition prevents the oxidation of iodine, which is the most important step for the synthesis of Thyroid Hormones. 
  4. Methimazole Also blocks the iodination tyrosine residues in thyroglobulin and also Coupling of diiodotyrosine and Monoiodotyrosines. 
  5. But it has no effect on the Peripheral circulating Thyroid Hormones. 

 

Discuss the side effects of antithyroids.      

  • Allergic reactions: Some people may have an allergic or hypersensitivity reaction to the antithyroid drugs, which can manifest as skin rash, pruritus, swollen glands, or sialadenitis.
  • Hepatotoxicity: Antithyroid drugs can cause liver damage or inflammation, which can lead to jaundice, elevated liver enzymes, or liver failure in rare cases.
  • Agranulocytosis: Antithyroid drugs can suppress the production of white blood cells, especially neutrophils, which are important for fighting infections. This can result in a condition called agranulocytosis, which is characterized by fever, sore throat, mouth ulcers, and increased susceptibility to infections.
  • Hypothyroidism: Antithyroid drugs can reduce the synthesis and release of thyroid hormones, which can lead to hypothyroidism or low thyroid function. This can cause symptoms such as fatigue, weight gain, cold intolerance, dry skin, hair loss, and depression.

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