Veterinary Physiology-III

ANTITHYROID COMPOUNDS

• Drugs or agents that depress the thyroid secretion are called as antithyroid compounds.
• The major cause of goitre in man and farm animals is a simple dietary iodine deficien­cy.
• Certain foodstuffs contain substances that inhibit thyroid activity. This goitrogenic activity is seen in feeding of cabbage, and other cruciferous plants, e.g., turnip, soybeans, linseed, peas, rapeseed, mustard seed and peanuts.
• A potent goitrogenic compound, goitrin, is responsible for most of the antithyroid activity of the rapeseed, mustard seed. However, the goitrogenecity of many plants is also due to the presence of other goitrogens such as thiocyanate.
• Since thiocyanate and other chemical radicals interfere with the trapping of iodine by the thyroid gland, the goitrogenicity of these radicals can be overcome by feeding excess iodide.
• It is more difficult to overcome the effect of goitrin and related compounds, since they interfere with the organic binding of iodine and not with the iodine trap.
• Other anti thyroid compounds includes sulfonamides, D‑amino salicylate and chlorpromazine.
• The compounds exerting the most potent antithyroid activity are the thiocarbamides, thiourea and thiouracil compounds which inhibit either the conversion of iodide to iodine or the organic binding of molecular iodine with tyrosine or the conversion of T₄ to T₃.
• These antithyroid substances are useful in the treatment of hyperthyroidism.