| Drugs that are known to inhibit the secretion of thyroid | | | | senses a decrease in the concentrations of thyroxine |
| hormones are generally known as antithyroid | | | | and triiodothyronine, it secretes more TSH. The higher |
| substances. Three of the best known antithyroid | | | | levels of TSH then causes overgrowth of the thyroid |
| substances are propylthiouracil, thiocyanate, and | | | | gland but still, the gland fails to produce enough |
| inorganic iodides. Each of this drugs blocks thyroid | | | | amounts of hormone. This leads to the formation of an |
| secretion in a different way and the different | | | | enlarged thyroid gland, also known as goiter. |
| mechanisms by which they do so are further | | | | Inorganic Iodides |
| explained. | | | | Inorganic iodides inhibit most of the activities only when |
| Propylthiouracil | | | | in very high concentrations, as in 100 times the normal |
| Propylthiouracil -- and its relatives, methimazole and | | | | plasma levels of iodides. This effect, however, remains |
| carbimazole -- decrease thyroid hormone formation by | | | | only for a few weeks. |
| preventing its formation from iodides and tyrosine. This | | | | This inhibitory effect of iodides on the thyroid gland is |
| drug acts by blocking the peroxidase enzyme that is | | | | due to the reduction in the rate of iodide trapping of |
| necessary in the first step of the formation of thyroid | | | | the gland, which also decreases the rate of production |
| hormones, which is the oxidation of iodine so it can | | | | of thyroxine and triiodothyronine. Even more so, the |
| combine to tyrosine. Propylthiouracil also blocks the | | | | normal endocytosis of colloid from the follicles by the |
| coupling of two iodinated tyrosines that are needed to | | | | thyroid glandular cells is paralyzed in the face of |
| form thyroxine or triiodothyronine, the two kinds of | | | | extremely high iodide concentrations. As this is the first |
| thyroid hormone. | | | | step in the release of thyroid hormones, thyroid |
| Propylthiouracil does not prevent the formation of | | | | hormone secretion is shut down almost immediately. |
| thyroglobulin. Since it only blocks the formation of | | | | The mechanisms by which inorganic iodides work on |
| thyroxine and triiodothyronine, the absence of both in | | | | the thyroid gland decreases the size of the thyroid and |
| the thyroglobulin can stimulate the pituitary gland to | | | | its blood supply due to its inactivity. This is the opposite |
| secrete more thyroid-stimulating hormone. This then | | | | of the previous two drugs, which only serve to |
| promotes the growth of glandular tissue despite its | | | | increase the size of the gland. |
| being unable to produce the two hormones and forms | | | | Propylthiouracil and Iodides in Thyroidectomy |
| a goiter.. | | | | Hyperthyroid patients must undergo several weeks of |
| Thiocyanate | | | | preparation before going under the knife. It was found |
| Thiocyanate ions decrease iodide trapping, the | | | | that these procedures could spell the difference |
| mechanism by which the thyroid gland acquires iodine | | | | between life and death for a patient and patient |
| from the circulating blood. The same pump that | | | | mortality has been greatly reduced since the |
| transports the iodide ions necessary for normal thyroid | | | | development of these preoperative procedures. |
| function can also pump thiocyanate ions, perchlorate | | | | Several weeks prior to the operation, the patient is |
| ions, and nitrate ions. Increased concentrations of | | | | administered with the antithyroid substance |
| thiocyanate ions then compete with the transport of | | | | propylthiouracil until such time that the basal metabolic |
| iodide ions into the thyroid cells. | | | | rate of the patient reaches normal levels. One to two |
| Like propylthiouracil, the decreased availability of iodide | | | | weeks before the operation, the patient receives high |
| does not stop the formation of thyroglobulin. | | | | doses of iodides, which causes the gland to recede in |
| Thiocyanate can only prevent the iodination of | | | | size and its blood supply to diminish. This is done to |
| thyroglobulin and in consequence, prevents the | | | | decrease the necessary amount of surgery -- an |
| formation of thyroid hormones. | | | | enlarged thyroid means a larger cut -- and to |
| As with propylthiouracil, when the pituitary gland | | | | decrease the amount of bleeding. |