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Questions and Answers
Which of the following best describes the mechanism of action of thioamide drugs in treating hyperthyroidism?
Which of the following best describes the mechanism of action of thioamide drugs in treating hyperthyroidism?
- They stimulate the release of preformed thyroid hormones from the thyroid gland.
- They enhance the conversion of $T_4$ to $T_3$ in peripheral tissues.
- They directly block the uptake of iodide into the thyroid gland.
- They inhibit thyroid peroxidase reactions, thereby reducing the synthesis of thyroid hormones. (correct)
A patient with Hashimoto's thyroiditis is being treated with levothyroxine. What laboratory finding would suggest that the dose of levothyroxine is too high?
A patient with Hashimoto's thyroiditis is being treated with levothyroxine. What laboratory finding would suggest that the dose of levothyroxine is too high?
- A suppressed level of thyroid-stimulating hormone (TSH). (correct)
- A decreased level of free $T_4$.
- Increased uptake of radioactive iodine by the thyroid gland.
- Elevated levels of thyroglobulin antibodies.
Which of the following is the primary mechanism by which T3 exerts its effects on target cells?
Which of the following is the primary mechanism by which T3 exerts its effects on target cells?
- Directly altering the permeability of the cell membrane to ions.
- Activation of cell surface receptors linked to second messenger systems.
- Inhibition of mitochondrial ATP production.
- Binding to nuclear receptors and altering gene transcription. (correct)
In the treatment of thyroid storm, why is potassium iodide administered after, rather than concurrently with, a thioamide drug?
In the treatment of thyroid storm, why is potassium iodide administered after, rather than concurrently with, a thioamide drug?
A patient with hyperthyroidism is being treated with propranolol. Which of the following effects of propranolol contributes most directly to the symptomatic relief of hyperthyroidism?
A patient with hyperthyroidism is being treated with propranolol. Which of the following effects of propranolol contributes most directly to the symptomatic relief of hyperthyroidism?
What is the most likely mechanism by which amiodarone induces thyroid dysfunction?
What is the most likely mechanism by which amiodarone induces thyroid dysfunction?
Radioactive iodine ($^{131}I$) is used to treat hyperthyroidism. What is the primary mechanism by which $^{131}I$ reduces thyroid activity?
Radioactive iodine ($^{131}I$) is used to treat hyperthyroidism. What is the primary mechanism by which $^{131}I$ reduces thyroid activity?
Which of the following drugs is the MOST appropriate initial treatment for a pregnant woman diagnosed with hyperthyroidism?
Which of the following drugs is the MOST appropriate initial treatment for a pregnant woman diagnosed with hyperthyroidism?
What is the primary advantage of using levothyroxine ($T_4$) over liothyronine ($T_3$) in the treatment of hypothyroidism?
What is the primary advantage of using levothyroxine ($T_4$) over liothyronine ($T_3$) in the treatment of hypothyroidism?
A patient with a history of Graves' disease develops exophthalmos. Which of the following treatments is MOST likely to improve this condition?
A patient with a history of Graves' disease develops exophthalmos. Which of the following treatments is MOST likely to improve this condition?
Which of the following mechanisms accounts for the cardiac-related adverse effects seen in thyrotoxicosis?
Which of the following mechanisms accounts for the cardiac-related adverse effects seen in thyrotoxicosis?
A patient is prescribed a drug that blocks the sodium/iodide symporter (NIS) in the thyroid gland. Which of the following effects would be MOST likely to occur?
A patient is prescribed a drug that blocks the sodium/iodide symporter (NIS) in the thyroid gland. Which of the following effects would be MOST likely to occur?
Which antithyroid drug also inhibits the peripheral conversion of $T_4$ to $T_3$?
Which antithyroid drug also inhibits the peripheral conversion of $T_4$ to $T_3$?
In a patient with hypothyroidism secondary to pituitary failure, what would be the expected laboratory findings?
In a patient with hypothyroidism secondary to pituitary failure, what would be the expected laboratory findings?
What is the primary reason why thyroid hormone levels are measured as 'free' $T_3$ and $T_4$ rather than total hormone levels?
What is the primary reason why thyroid hormone levels are measured as 'free' $T_3$ and $T_4$ rather than total hormone levels?
Why is it important to monitor for agranulocytosis in patients taking thioamide drugs?
Why is it important to monitor for agranulocytosis in patients taking thioamide drugs?
The use of which medication is contraindicated with thyroid hormone replacement medication?
The use of which medication is contraindicated with thyroid hormone replacement medication?
Which of the following best describes the role of thyroxine-binding globulin (TBG) in thyroid hormone homeostasis?
Which of the following best describes the role of thyroxine-binding globulin (TBG) in thyroid hormone homeostasis?
A patient with a known iodine deficiency develops a goiter. What is the primary mechanism leading to goiter formation in this scenario?
A patient with a known iodine deficiency develops a goiter. What is the primary mechanism leading to goiter formation in this scenario?
T4 is more active than T3
T4 is more active than T3
Which of the following converts T4 to T3 in the plasma?
Which of the following converts T4 to T3 in the plasma?
Which of the following is removed by the thyroid from circulation in the diet?
Which of the following is removed by the thyroid from circulation in the diet?
Which of the following pumps iodide into the thyroid gland from the blood?
Which of the following pumps iodide into the thyroid gland from the blood?
Which of the following converts iodide to iodine?
Which of the following converts iodide to iodine?
What does the iodination of tyrosine by iodine form?
What does the iodination of tyrosine by iodine form?
What is T3 formed by?
What is T3 formed by?
What is T4 formed by?
What is T4 formed by?
Which of the following carries T3 and T4 in plasma?
Which of the following carries T3 and T4 in plasma?
What is the function of rT3?
What is the function of rT3?
How is rT3 formed?
How is rT3 formed?
Which of the following is described:
- synthetic T4, the most common form given
- converted to T3 in the body
- Drug has a long half-life of 7 days?
Which of the following is described:
- synthetic T4, the most common form given
- converted to T3 in the body
- Drug has a long half-life of 7 days?
Which of the following is described as synthetic T3 with a short half-life of 24 hours?
Which of the following is described as synthetic T3 with a short half-life of 24 hours?
Which of the following refers to a fixed dose combination of thyroxine and liothyronine?
Which of the following refers to a fixed dose combination of thyroxine and liothyronine?
Which of the following are thioamides?
Which of the following are thioamides?
What is the mechanism of action (MOA) of thioamides?
What is the mechanism of action (MOA) of thioamides?
What is the specific action of thioamides?
What is the specific action of thioamides?
What is the most common ADR of thioamides?
What is the most common ADR of thioamides?
What is the most dangerous adverse drug reaction (ADR) of thioamides?
What is the most dangerous adverse drug reaction (ADR) of thioamides?
Which of the following are anion inhibitors?
Which of the following are anion inhibitors?
What is the mechanism of action (MOA) of anion inhibitors?
What is the mechanism of action (MOA) of anion inhibitors?
What is the mechanism of action (MOA) of iodides?
What is the mechanism of action (MOA) of iodides?
What is radioactive ablation?
What is radioactive ablation?
Which of the following is a radioactive iodide taken PO?
Which of the following is a radioactive iodide taken PO?
What is the basis for the use of radioactive iodine?
What is the basis for the use of radioactive iodine?
What is the most common cause of hypothyroidism?
What is the most common cause of hypothyroidism?
Which of the following is indicative of hypothyroidism?
Which of the following is indicative of hypothyroidism?
Which of the following could be a cause of hypothyroidism? (Select all that apply)
Which of the following could be a cause of hypothyroidism? (Select all that apply)
What is the treatment for Hashimoto's Thyroiditis (or any hypothyroidism)?
What is the treatment for Hashimoto's Thyroiditis (or any hypothyroidism)?
What is the primary cause of hyperthyroidism?
What is the primary cause of hyperthyroidism?
What are the common treatments for hyperthyroidism? (Select all that apply)
What are the common treatments for hyperthyroidism? (Select all that apply)
What is a goiter?
What is a goiter?
What is a thyroid storm?
What is a thyroid storm?
Which of the following is used to treat thyroid storm? (Select all that apply)
Which of the following is used to treat thyroid storm? (Select all that apply)
What is the role of beta blockers such as propranolol and esmolol in treating thyroid storm?
What is the role of beta blockers such as propranolol and esmolol in treating thyroid storm?
What is the role of potassium iodide in treating thyroid storm?
What is the role of potassium iodide in treating thyroid storm?
What is the role of propylthiouracil in a thyroid storm?
What is the role of propylthiouracil in a thyroid storm?
Which of the following will block the conversion of T4 to T3 in the treatment of a thyroid storm?
Which of the following will block the conversion of T4 to T3 in the treatment of a thyroid storm?
Which of the following thyroid replacement drugs is from cow thyroids?
Which of the following thyroid replacement drugs is from cow thyroids?
Which of the following is a recombinant human TSH replacement hormone?
Which of the following is a recombinant human TSH replacement hormone?
What is the mechanism of action (MOA) of methimazole?
What is the mechanism of action (MOA) of methimazole?
Which of the following has the described mechanism of action (MOA):
- block iodine organification
- inhibit peripheral deiodination?
Which of the following has the described mechanism of action (MOA):
- block iodine organification
- inhibit peripheral deiodination?
Which of the following iodides inhibit organification and hormone release?
Which of the following iodides inhibit organification and hormone release?
Which of the following drugs are inhibited by targets of the sodium/iodide symporter? (Select all that apply)
Which of the following drugs are inhibited by targets of the sodium/iodide symporter? (Select all that apply)
Which of the following are inhibited by drugs that target thyroidal peroxidase?
Which of the following are inhibited by drugs that target thyroidal peroxidase?
Which of the following are inhibited by drugs that target 5'-deiodinase? (Select all that apply)
Which of the following are inhibited by drugs that target 5'-deiodinase? (Select all that apply)
What is the function of T3?
What is the function of T3?
Which of the following drugs accumulates after oral dosing in the thyroid gland?
Which of the following drugs accumulates after oral dosing in the thyroid gland?
Which of the following drugs has the described major adverse drug reactions (ADRs):
- crosses the placental barrier
- can cause hypothyroidism in the fetus
- must not be used in pregnancy
- congenital malformations have been reported?
Which of the following drugs has the described major adverse drug reactions (ADRs):
- crosses the placental barrier
- can cause hypothyroidism in the fetus
- must not be used in pregnancy
- congenital malformations have been reported?
What is the FDA Pregnancy Category of Methimazole?
What is the FDA Pregnancy Category of Methimazole?
Methimazole does not cross into mother’s milk and is safe for nursing mothers
Methimazole does not cross into mother’s milk and is safe for nursing mothers
What is myxedema coma?
What is myxedema coma?
What are the adverse drug reactions (ADRs) of levothyroxine? (Select all that apply)
What are the adverse drug reactions (ADRs) of levothyroxine? (Select all that apply)
What antiarrhythmic agent can potentially cause either hypothyroidism or hyperthyroidism?
What antiarrhythmic agent can potentially cause either hypothyroidism or hyperthyroidism?
What drugs inhibit the release of preformed thyroid hormone?
What drugs inhibit the release of preformed thyroid hormone?
What is the drug of choice for hyperthyroidism in pregnancy?
What is the drug of choice for hyperthyroidism in pregnancy?
Flashcards
What is T4?
What is T4?
Tetraiodothyronine, a thyroid hormone.
What is T3?
What is T3?
Triiodothyronine, a thyroid hormone.
What is the sequence of thyroid hormone release?
What is the sequence of thyroid hormone release?
Hypothalamus releases TRH, pituitary releases TSH, thyroid releases T4 and T3.
What is 5' - deiodinase?
What is 5' - deiodinase?
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What role does the thyroid play with iodide?
What role does the thyroid play with iodide?
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What is thyroidal peroxidase?
What is thyroidal peroxidase?
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What is Thyroxine Binding Globulin (TBG)?
What is Thyroxine Binding Globulin (TBG)?
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What is Thyrotropin Releasing Hormone (TRH)?
What is Thyrotropin Releasing Hormone (TRH)?
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What is Thyroid Stimulating Hormone (TSH)?
What is Thyroid Stimulating Hormone (TSH)?
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Which forms of T4 and T3 are active?
Which forms of T4 and T3 are active?
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What is levothyroxine?
What is levothyroxine?
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What is liothyronine?
What is liothyronine?
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What do thioamides do?
What do thioamides do?
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What is Methimazole?
What is Methimazole?
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What do anion inhibitors do?
What do anion inhibitors do?
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What are common causes of hypothyroidism?
What are common causes of hypothyroidism?
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What is a common cause of hyperthyroidism?
What is a common cause of hyperthyroidism?
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What is Thyroid Storm?
What is Thyroid Storm?
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What is role of Radioactive iodine?
What is role of Radioactive iodine?
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What is the function of Iodides?
What is the function of Iodides?
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Study Notes
Thyroid Hormones
- T4 is tetraiodothyronine, also known as L-thyroxine
- T3 is triiodothyronine
- The numbers 3 and 4 indicate the number of iodine molecules in each hormone
- These hormones are synthesized from two amino acids linked together, with iodine attached
- T3 is more biologically active compared to T4
- The Hypothalamus releases Releasing Factors to trigger the release of thyroid hormones
- In response to Releasing Factors, the Pituitary releases Thyroid Stimulating Hormone (TSH) into the blood
- TSH travels to the thyroid gland
- TSH stimulates the thyroid to release T4 and T3
- T4 in the plasma gets converted to T3 by 5'deiodinase
- Target cells take up T3, which then binds to nuclear receptors
- T3 binding to nuclear receptors alters gene transcription
- These actions on the DNA level have widespread metabolic effects across the body
Iodide
- The thyroid removes iodide from circulation, which comes from the diet
- The sodium/iodide symporter (NIS) actively transports iodide into the thyroid gland from the blood
- Thyroidal peroxidase converts iodide into iodine
- Iodine then iodinates tyrosine, to form monoiodotyrosine (MIT) and diiodotyrosine (DIT)
- T3 is formed from MIT and DIT
- T4 is formed from two DIT molecules
- Thyroxine binding globulin (TBG) is the plasma protein that carries both T3 and T4 in the blood
Hormone Levels
- T4 is present in micrograms per deciliter (ug/dL), while T3 is in nanograms per deciliter (ng/dL)
- T3 is more potent, despite its lower concentration
- For comparison, T4 blood levels are between 4.8 - 10.4 mcg/dL, and T3 levels are between 0.06 - 0.181 mcg/dL
- T3 is 3-4 times more potent than T4
Regulation
- 5'-deiodinase is the enzyme that converts T4 to T3
- A feedback mechanism regulates the thyroid gland
- The hypothalamus releases thyrotropin releasing hormone (TRH)
- TRH stimulates the pituitary to release TSH
- TSH stimulates the thyroid gland to release T4 and T3
Pharmacology of Thyroid Hormones
- Only the free forms of T4 and T3 are active
- Both T4 and T3 hormones enter cells like steroids
- 5'-deiodinase converts T4 to T3 inside cells
- T3 binds to specific receptors in the nucleus
- Nuclear receptor activation leads to increased RNA formation and protein synthesis
- This explains the lag times observed with thyroid hormones and drugs
- Thyroid hormones regulate a wide array of functions due to their effect on gene transcription
- Conversion of T4 to T3 happens both inside and outside of cells
Thyroid Hormones as Drugs
- Thyroid hormone drugs are available as synthetic or animal-derived forms
- Desiccated thyroid is of animal origin but is rarely used
- Animal-derived material is dangerous due to protein antigenicity
- Levothyroxine is synthetic T4, is the most common form, with a long half-life of 7 days and is converted to T3
- Liothyronine is synthetic T3 and has a shorter half-life of 24 hours
- Liotrix is a combination drug with a fixed dose of thyroxine and liothyronine
- Levothyroxine is the most widely used thyroid drug
Antithyroid Drugs: Thioamides
- Examples include methimazole and propylthiouracil
- They inhibit thyroid peroxidase reactions
- They prevent hormone synthesis by blocking iodine incorporation onto the amino acid complex
- The most common adverse drug reaction (ADR) is maculopapular pruritic rash
- Agranulocytosis is the most dangerous ADR
Antithyroid Drugs: Anion Inhibitors
- Examples include perchlorate, pertechnetate, and thiocyanate
- They function by blocking iodide uptake by the thyroid gland
Antithyroid Drugs: Iodides
- Iodides inhibit organification and hormone release
- They can impair thyroid gland function in instances of excess iodine
Antithyroid Drugs: Radioactive Iodine
- Radioactive iodine-125 is administered orally
- The thyroid gland concentrates it, and beta rays destroy the gland tissue
- Radioactive iodine destroys the thyroid gland, known as radioactive ablation
Clinical Pharmacology: Hypothyroidism
- Characterized by low free T4 and elevated TSH levels
- Hashimoto's thyroiditis is the most common cause
- It is an immune disorder in which the patient develops thyroid-destroying antibodies
- The thyroid does not produce T4 and T3, and the pituitary continuously sends TSH signals
- Treatment includes thyroid replacement drugs such as levothyroxine
Clinical Pharmacology: Hyperthyroidism
- Graves' disease is a common cause
- Patients produce TSH-like antibodies that stimulate the thyroid gland
- These antibodies acts as agonists and stimulate T4 and T3 release
- Elevated T4, T3, and free hormone levels, coupled with low TSH levels indicate hyperthyroidism
- Treatment consists of antithyroid drugs or thyroid gland removal by partial or complete thyroidectomy
- Radioactive iodine is also used
Thyroid-Related Conditions
- Goiter describes a nodule in the neck over the thyroid gland, can occur in either hyper- or hypothyroidism
- Thyroid storm is a life-threatening form of thyrotoxicosis
- Propranolol or esmolol (beta blockers) are administered to control the cardiac effects of excess T4 and T3
- Potassium iodide helps shut down the thyroid gland
Other drugs affecting the thyroid
- Propythiouracil stops hormone synthesis
- Hydrocortisone and propranolol both block the conversion of T4 to T3
Summary of Thyroid-Related Drugs:
- Thyroid replacement hormones: levothyroxine (T4), liothyronine (T3), liotrix (T4 plus T3), and thyroid desiccated, activate gene expression in cells for protein synthesis
- Thyrotropin: recombinant human TSH
- Thioamides: methimazole and propylthiouracil, inhibit thyroid peroxidase, block iodine organification, and inhibit peripheral deiodination
- Iodides: Lugol solution and potassium iodide, inhibit organification and hormone release
- Beta blockers: propranolol and esmolol, inhibit conversion of T4 to T3
- Radioactive iodine: 131I iodine, radiation destruction for thyroid cancer treatment
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