QS Study

Antithyroid substances

Drugs that suppress thyroid secretion are called antithyroid substances. It is a hormone antagonist acting upon thyroid hormones. The best known of these substances are –

  1. Thiocyanate,
  2. Propylthiouracil,
  3. High concentration of inorganic iodides.

Mode of action

(i) Thiocyanate – Thiocyanate Ions Decrease Iodide Trapping.

  • It decreases the iodide trapping by decreasing the rate at which iodide is pumped into follicular cells.
  • This reduces the availability of iodide for the formation of the thyroid hormones.
  • Lack of thyroid hormone causes the thyroid gland to become enlarged (goiter).

(ii) Propylthiouracil – It prevents the formation of thyroid hormone from iodides and tyrosine.

A similar compound such as methimazole, carbimazole)

  • Partly the peroxidase enzyme that is required for the block iodination of tyrosine amino acid.
  • It blocks the coupling of two iodinated tyrosines.
  • Thus it prevents the formation of thyroid hormones.
  • The mechanism of this is partly to block the peroxidase enzyme that is required for iodination of tyrosine and partly to block the coupling of two iodinated tyrosines to form thyroxine or triiodothyronine.

(iii) Inorganic iodides: Because iodides in high concentrations decrease all phases of thyroid activity, they slightly decrease the size of the thyroid gland and especially decrease its blood

  • In high conc. iodine decreases all phases of thyroid activity.
  • They decrease the size of the gland and especially decrease its blood supply.