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Thyroid Stimulating and Inhibiting Drugs

Thyroid Stimulating and Inhibiting Drugs. These drugs are hard to use, because they have a very narrow therapeutic window, and it is easy ​to induce iatrogenic thyrotoxicosis . Treatment failure is usually due to non-compliance, since even small deviations make a difference .

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Thyroid Stimulating and Inhibiting Drugs

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  1. Thyroid Stimulating and Inhibiting Drugs

  2. These drugs are hard to use, because they have a very narrow therapeutic window, and it is easy ​to induce iatrogenic thyrotoxicosis.

  3. Treatment failure is usually due to non-compliance, since even small deviations make a difference.

  4. Treatment of Hyperthyroidism • --Thionamides: Propulthiouracil (PTU) and Methimazole (MMI) • PTU and MMI are competitive inhibitors of thyroid peroxidase, so they block organification ​of thyroglobulin by iodine.

  5. They also have immunosuppressive effects, which contributes to their efficacy against Graves’ Disease.

  6. Thionamides are used to treat thyrotoxicosis and Graves’ Disease, but are inappropriate for ​thyroiditis-related hyperthyroidism, which is self-limited. • MMI has a longer half life than PTU.

  7. PTU partially inhibits peripheral conversion of T4 to T3, but MMI does not. • Both PTU and MMI cross the placenta and are found in breast milk.

  8. Toxicities: • ​--reversible hypothyroidism • ​--agranulocytosis (rare, but life-threatening)!!

  9. --Radioactive Iodine: 131Iodine • Radioactive Iodine has a very short radiation path and is taken right to the thyroid.

  10. Radioactive Iodine is primarily used against Toxic Adenoma, Toxic Multinodular Goiter, and Graves’. • It cures over 99% of patients within three doses.

  11. Radioactive Iodine is inappropriate for thyroiditis-related hyperthyroidism, and during pregnancy. • Toxicities:​--permanent hypothyroidism

  12. --Beta-Blockers: Propranolol and Atenolol • These drugs are used to block the adrenergic, sympathetic manifestations of hyperthyroidism, ​such as tremor, palpitations, and anxiety.

  13. They are used in the management of severe ​thyrotoxicosis, especially if there are cardiac manifestations such as tachyarrythmias. • Propranolol, like PTU, partially inhibits peripheral conversion of T4 to T3.

  14. Treatment of Hypothyroidism • L-thyroxine (T4) is the main treatment for hypothyroidism. • It is well absorbed except in the presence of iron supplementation and intestinal problems.

  15. T4 converts to T3 except in the presence of starvation, systemic illness, propranolol, or amiodarone.

  16. Binding of plasma protein to thyroid hormone prolongs half life. • T4 has a half life of a week, T3 has a half life of a day. • This is why we give T4 instead of T3.

  17. Drugs such as Phenytoin and Rifampin, as well as pregnancy, accelerate clearance of hormones ​and necessitate higher doses of T4. • T4 dosing is also dependent on age and weight.

  18. When using L-thyroxine, monitor patients for sign of thyrotoxicosis, and regularly measure TSH ​to make sure it is staying low enough.

  19. Toxicities: • ​--reversible hyperthyroidism • ​--exacerbation of ischemic heart disease (atrial fibrillation, increased inotropy) • ​--provoking adrenal insufficiency by increasing metabolism of cortisol​ • ​--osteoporosis, transient hair loss

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