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Questions and Answers
What is the primary effect of the β-particles emitted by radioactive iodine?
What is the primary effect of the β-particles emitted by radioactive iodine?
Which condition is a contraindication for the use of radioactive iodine?
Which condition is a contraindication for the use of radioactive iodine?
What is an advantage of using radioactive iodine treatment for hyperthyroidism?
What is an advantage of using radioactive iodine treatment for hyperthyroidism?
In what situation would β-blockers be used in relation to thyroid issues?
In what situation would β-blockers be used in relation to thyroid issues?
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What is a common disadvantage of radioactive iodine treatment?
What is a common disadvantage of radioactive iodine treatment?
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What is required for thyroxine (T4) to effectively bind to its nuclear receptor?
What is required for thyroxine (T4) to effectively bind to its nuclear receptor?
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Which of the following treatments is recommended for myxedema coma?
Which of the following treatments is recommended for myxedema coma?
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What initial dose of levothyroxine is recommended for older patients without cardiac disease?
What initial dose of levothyroxine is recommended for older patients without cardiac disease?
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What regulates the synthesis of thyroid hormones in the thyroid gland?
What regulates the synthesis of thyroid hormones in the thyroid gland?
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The process of iodide trapping in the thyroid gland is primarily facilitated by which mechanism?
The process of iodide trapping in the thyroid gland is primarily facilitated by which mechanism?
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Which class of drugs is used to reduce the synthesis or release of thyroid hormones?
Which class of drugs is used to reduce the synthesis or release of thyroid hormones?
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What side effect is associated with levothyroxine therapy?
What side effect is associated with levothyroxine therapy?
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What is the result of the coupling process in thyroid hormone synthesis?
What is the result of the coupling process in thyroid hormone synthesis?
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What condition results from a deficiency of thyroid hormones in children?
What condition results from a deficiency of thyroid hormones in children?
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Which mechanism of action is attributed to thioamides in treating hyperthyroidism?
Which mechanism of action is attributed to thioamides in treating hyperthyroidism?
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What should be administered after 48-72 hours of intravenous liothyronine treatment for myxedema coma?
What should be administered after 48-72 hours of intravenous liothyronine treatment for myxedema coma?
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Which of the following compounds can inhibit the oxidation and iodination of iodide ions?
Which of the following compounds can inhibit the oxidation and iodination of iodide ions?
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What is one of the actions of propylthiouracil, a thioamide?
What is one of the actions of propylthiouracil, a thioamide?
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What primarily occurs during the release of thyroid hormones?
What primarily occurs during the release of thyroid hormones?
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What effect does propylthiouracil have on thyroid hormone levels?
What effect does propylthiouracil have on thyroid hormone levels?
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Which thyroid hormone is more potent in its biological effects?
Which thyroid hormone is more potent in its biological effects?
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What is the primary mechanism of action of propylthiouracil?
What is the primary mechanism of action of propylthiouracil?
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Which of the following is a rare but dangerous side effect associated with thioamide therapy?
Which of the following is a rare but dangerous side effect associated with thioamide therapy?
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How often should propylthiouracil be administered due to its short half-life?
How often should propylthiouracil be administered due to its short half-life?
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Lugol's iodine is primarily used in what clinical scenario?
Lugol's iodine is primarily used in what clinical scenario?
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What is one of the primary adverse effects associated with chronic overdose of iodide?
What is one of the primary adverse effects associated with chronic overdose of iodide?
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What is the therapeutic use of radioactive iodine (I)131?
What is the therapeutic use of radioactive iodine (I)131?
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Which of the following adverse effects is associated with iodide treatment during pregnancy?
Which of the following adverse effects is associated with iodide treatment during pregnancy?
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What role does potassium iodide (KI) play in clinical treatment?
What role does potassium iodide (KI) play in clinical treatment?
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Study Notes
Pharmacology of Thyroid Gland
- The thyroid gland secretes thyroxine (T4), triiodothyronine (T3), and calcitonin
- Thyroid follicular cells have a specialized mechanism for producing thyroid hormones
- Thyroid hormone synthesis is regulated by TSH released from the anterior pituitary
- TSH release is inhibited by free thyroid hormone levels
- C cells of the thyroid secrete calcitonin, which regulates calcium metabolism
Thyroid Hormone Effects
- Thyroid hormone is synergistic with growth hormone
- Thyroid hormone increases tissue oxygen consumption, basal metabolic rate, and body temperature
- Thyroid hormone increases cardiac output and ventilation
- Thyroid hormone facilitates central nervous system (CNS) maturation and maintenance
Thyroid Hormone Synthesis
- Iodide Trapping: Active transport of iodide ions into follicular cells via a sodium/iodide symporter. This can be inhibited by thiocyanates and perchlorates.
- Oxidation and Iodination: Iodide is oxidized to iodine by peroxidase enzyme. Iodine combines with tyrosine residues of thyroglobulin, forming monoiodotyrosine (MIT) and diiodotyrosine (DIT).
- Coupling: Two DIT molecules combine to form thyroxine (T4), and one MIT and one DIT molecule combine to form triiodothyronine (T3).
Hormone Release
- Thyroid hormone release is controlled by TSH
- The process involves endocytosis and proteolysis of iodinated thyroglobulin
- Proteolysis is inhibited by high levels of intrafollicular iodide
Peripheral Conversion of T4 to T3
- Most of the hormone released from the thyroid gland is T4, which is less potent than T3
- Peripheral conversion of T4 to T3 is inhibited by propylthiouracil, iopanoic acid, propranolol, and glucocorticoids
Differences Between T3 and T4
| Feature | T3 (Triiodothyronine) | T4 (Thyroxine) |
|---|---|---|
| Formation | DIT + MIT | DIT + DIT |
| Onset of Action | Relatively rapid (half-life of 1 day) | Slower (half-life of 7 days) |
| Potency | More potent | Less potent |
| Use | Treating myxedema coma | Treating myxedema coma and regular myxedema treatment |
Mechanism of Action
- Thyroid hormones' mechanism of action is similar to steroid hormones
- Thyroxine (T4) needs conversion to T3 inside the cell to bind to nuclear receptors
- T3 and T4 enter the cell and are converted to T3 and bind to nuclear receptors
- This leads to activation of different genes and the synthesis of various proteins
- The effects are diverse
Hyperthyroidism and Hypothyroidism
-
Hyperthyroidism (Thyrotoxicosis): Increased basal metabolic rate, decreased cholesterol and triglycerides, increased glycogenolysis and gluconeogenesis, leading to hyperglycemia, negative nitrogen balance. Increased heart rate, stroke volume, and cardiac output.
-
Hypothyroidism (Myxedema): Decreased basal metabolic rate, increased cholesterol and triglycerides, hypoglycemia, positive nitrogen balance. Decreased heart rate, stroke volume, and cardiac output.
CNS, Musculoskeletal, Gastrointestinal, and Other Features
- Hyperthyroidism: nervousness, anxiety, weakness, muscle fatigue, increased deep tendon reflexes, increased appetite, diarrhea, anaemia, menstrual irregularities
- Hypothyroidism: lethargy, mental retardation (cretinism), stiffness, muscle fatigue, decreased appetite, constipation, ascites, anaemia, menstrual irregularities, puffy face, large tongue, etc. Other features are mentioned.
Therapeutic Uses
- Cretinism and Myxedema: Treatment with levothyroxine sodium, starting early and in full replacement doses.
- Myxedema Coma: A medical emergency requiring IV levothyroxine, followed by oral levothyroxine.
Levothyroxine (T4)
- Has all the actions of endogenous thyroxine
- Orally administered, 1.6-1.7 mcg/kg/day
- Initial dose lower in older patients with cardiac disease
- Taken 30 minutes before eating
Liothyronine (T3)
- IV treatment of choice for myxedema coma
- After improvement, switch to oral levothyroxine
- Mixture with levothyroxine (Liotrix) is better tolerated and has a longer half-life
Antithyroid Drugs
- Reduce thyroid hormone levels by inhibiting synthesis or release
- Used in hyperthyroidism
- Thionamides—the most common class; inhibit the enzyme peroxidase and the iodination of tyrosine residues -Examples include propylthiouracil, methimazole, carbimazole
- Hormone-release inhibitors — examples are Iodine, potassium iodide
- Thyroid tissue-destroying agent—example is radioactive iodine(I131)
- Other drugs — Propranolol, diltiazem, dexamethasone
Radioactive Iodine
- Concentrates in thyroid and emits radiation, destroying follicular cells, leading to fibrosis and fixing hyperthyroidism
- Used in persistent hyperthyroidism when surgery is not indicated or feasible, and in cases of adenoma or carcinoma.
- Contraindicated in pregnancy, children and Graves' disease related to eye manifestations
Beta-Adrenoceptor Blockers (β-Blockers)
- Not strictly antithyroid drugs but help manage symptoms (e.g., tachycardia, palpitation, tremors)
- Propranolol inhibits peripheral conversion of T4 to T3
- Used to control symptoms of thyrotoxicosis initially and in thyrotoxic crisis. Also useful pre-operatively.
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Description
This quiz covers essential concepts related to the use of radioactive iodine in treating thyroid conditions. It includes questions about the effects, contraindications, and advantages of this treatment, as well as related hormonal therapies. Test your knowledge of endocrinology and thyroid management practices.
