Thyroid Hormones and Anti-Thyroid Agents

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Questions and Answers

What are the primary hormones released by the thyroid gland?

The primary hormones released by the thyroid gland are tetraiodothyronine (T4) and triiodothyronine (T3).

What does the term 'goiter' refer to?

Goiter refers to the enlargement of the thyroid gland from any cause.

What is the role of calcitonin in the body?

Calcitonin is needed for the regulation of calcium metabolism.

Why is iodine important for thyroid hormone synthesis?

Iodine is essential for the synthesis of thyroid hormones T4 and T3. Signup and view all the answers

How do thyroid hormones affect energy regulation in the body?

Thyroid hormones regulate the resting metabolic rate of most tissues. Signup and view all the answers

What are the potential consequences of thyroid hormone deficiency in early life?

Thyroid hormone deficiency early in life can lead to irreversible mental retardation and dwarfism. Signup and view all the answers

What are common problems associated with thyroid gland activity?

Common problems include underactivity (hypothyroidism) and overactivity (hyperthyroidism). Signup and view all the answers

How does hyperactivity of the thyroid gland affect the body?

Hyperactivity of the thyroid gland produces effects similar to sympathetic nervous system overactivity. Signup and view all the answers

What are the normal monitoring levels for Free T4 and TSH in thyroid therapy?

Free T4 and TSH levels must be within normal levels. Signup and view all the answers

What is the most prescribed thyroid preparation for hypothyroidism?

Levothyroxine is the most prescribed preparation. Signup and view all the answers

Why is liothyronine not recommended for routine replacement therapy?

Liothyronine has a shorter half-life and necessitates multiple daily dosing. Signup and view all the answers

What are some disadvantages of using dessicated thyroid for treatment?

Disadvantages include protein antigenicity and variable hormone concentration. Signup and view all the answers

How often is levothyroxine administered to patients?

Levothyroxine is administered once daily orally. Signup and view all the answers

What risks are associated with using liothyronine in patients with cardiac disease?

Liothyronine poses a greater risk of cardio toxicity. Signup and view all the answers

What is the shelf life of synthetic thyroid preparations like levothyroxine?

The shelf life is about 2 years when stored properly. Signup and view all the answers

What is the toxicity relationship with levothyroxine?

Toxicity seen with levothyroxine is directly related to hormone level. Signup and view all the answers

What is the primary action of thioamides in thyroid hormone synthesis?

Thioamides primarily inhibit the coupling of iodotyrosines, blocking thyroid hormone synthesis. Signup and view all the answers

What is a notable adverse effect associated with thioamides?

A common adverse effect is a maculopapular pruritic rash. Signup and view all the answers

How do anion inhibitors affect thyroid function?

Anion inhibitors such as perchlorate competitively block iodide uptake by the thyroid gland. Signup and view all the answers

What is a specific use of potassium perchlorate?

Potassium perchlorate is useful for treating iodide-induced hyperthyroidism, like that caused by amiodarone. Signup and view all the answers

What role do iodides play in thyroid therapy?

Iodides inhibit organification and hormone release, and can reduce the size and vascularity of the thyroid gland. Signup and view all the answers

What is the maximum daily dose of iodides recommended?

Iodides should be administered at less than 6 mg daily. Signup and view all the answers

Why might cross-sensitivity be a concern with thioamide medications?

Cross-sensitivity within thioamides may lead to severe reactions if a patient switches between them. Signup and view all the answers

What is a significant rare adverse effect of potassium perchlorate?

A significant rare adverse effect is aplastic anemia. Signup and view all the answers

What is iodide trapping and which substances can inhibit it?

Iodide trapping is the transport of iodide into the thyroid gland, and it can be inhibited by anions such as SCN^- and TCO4^-. Signup and view all the answers

What role does thyroidal peroxidase play in iodine metabolism?

Thyroidal peroxidase oxidizes iodide to iodine, a step that can be inhibited by thioamides and excess iodide. Signup and view all the answers

What is iodine organification and what compounds are formed during this process?

Iodine organification refers to the iodinates of tyrosine residues in thyroglobulin, forming monoiodotyrosine (MIT) and diiodotyrosine (DIT). Signup and view all the answers

How are T4 and T3 formed from iodotyrosines?

Two molecules of DIT combine to form L-thyroxine (T4), while one molecule of MIT and one molecule of DIT form triiodothyronine (T3). Signup and view all the answers

What inhibits the release of thyroid hormones from thyroglobulin?

High levels of iodide within the gland can inhibit the proteolysis required for the release of thyroid hormones from thyroglobulin. Signup and view all the answers

What is the primary transport protein for T4 and T3 in the plasma?

T4 and T3 are primarily transported in plasma bound to thyroxine binding globulin (TBG). Signup and view all the answers

How is thyroxine metabolized in the body?

Thyroxine is metabolized mainly through deiodination, leading to the formation of T3, and can also be inactivated via deamination or conjugation. Signup and view all the answers

Which physiological factors can affect the transport of T4 and T3?

The transport of T4 and T3 via TBG can be inhibited by drugs, physiologic conditions, and pathologic states. Signup and view all the answers

What are the main problems associated with the use of radiation in treating hyperthyroidism?

Radiation can induce genetic damage, leukemia, and neoplasms, particularly in patients over 40 years old. Signup and view all the answers

Why are beta blockers used in the treatment of thyrotoxicosis?

Beta blockers mimic sympathetic stimulation and help alleviate symptoms associated with thyrotoxicosis. Signup and view all the answers

What is a thyroid storm and why is it considered life threatening?

A thyroid storm is a sudden acute exacerbation of all symptoms of thyrotoxicosis, which can be life threatening due to its severity. Signup and view all the answers

List two treatments administered during a thyroid storm.

Propranolol and saturated potassium iodide are two treatments given during a thyroid storm. Signup and view all the answers

What surgical procedure is recommended for women with thyrotoxicosis during pregnancy?

Subtotal thyroidectomy is recommended for women of child-bearing age with thyrotoxicosis. Signup and view all the answers

What role do corticosteroids play in the treatment of thyroid storm?

Corticosteroids like hydrocortisone protect against shock and block the conversion of T4 to T3. Signup and view all the answers

What is the significance of using iodinated contrast in the treatment of thyrotoxicosis?

Iodinated contrast blocks the peripheral conversion of T4 to T3, helping to manage hormone levels. Signup and view all the answers

Why should radiation not be given to pregnant women or nursing mothers?

Radiation poses significant risks to the developing fetus and nursing infants. Signup and view all the answers

Study Notes

Introduction

The normal adult thyroid gland weighs between 15-25 grams and is butterfly-shaped, located at the front of the neck.
Enlargement of the thyroid due to various causes is termed "Goiter."
Primary hormones released by the thyroid include tetraiodothyronine (thyroxine, T4), triiodothyronine (T3), and calcitonin.
T3 and T4 are crucial for growth, development, energy regulation, and maintaining normal body temperature.
Calcitonin helps regulate calcium metabolism and is essential for skeletal and reproductive health.
Deficiency in T4 or T3 during infancy can lead to irreversible mental retardation and dwarfism.
Thyroid hormones influence other hormones, including catecholamines, estrogens, and cortisol, and regulate the metabolic rate of most tissues.

Hormone Synthesis and Release

Iodide is actively transported into the thyroid gland, known as iodide trapping; this process can be inhibited by certain anions.
Iodide is converted to iodine by thyroidal peroxidase, which is inhibited by thioamides and excess iodide.
Iodine iodinates tyrosine residues in thyroglobulin, resulting in monoiodotyrosine (MIT) and diiodotyrosine (DIT).
Two DIT molecules form T4, while one MIT and one DIT yield T3.
T4 and T3 are released from thyroglobulin through exocytosis; excess MIT and DIT are deiodinated for iodine recycling.
The ratio of T4 to T3 released is 5:1, with most circulating T3 deriving from peripheral conversion of T4.

Transportation and Metabolism

T4 and T3 mostly bind irreversibly to thyroxine-binding globulin (TBG) in plasma, with free forms constituting 0.04% of T4 and 0.4% of T3.
Major metabolic pathways for T4 include deiodination, leading to the active T3 form, which is 3-4 times more potent than T4.
Routine monitoring of free T4 and TSH levels is crucial, typically reaching steady state 6-8 weeks post-therapy initiation.

Thyroid Preparations

Synthetic thyroid preparations like levothyroxine, liothyronine, and liotrix are used for hypothyroidism, with levothyroxine being the most commonly prescribed due to its stability and cost-effectiveness.
Levothyroxine has a long half-life of 7 days and is administered once daily; it converts intracellularly to T3.
Liothyronine has a shorter half-life (about 24 hours) and is not recommended for routine use due to multiple daily dosing and potential cardio toxicity.
Animal-derived preparations such as desiccated thyroid have drawbacks, including variable hormone concentration and antigenicity.

Anti-Thyroid Agents

Thioamides inhibit coupling of iodotyrosines and peripheral deiodination of T4 to T3, taking 3-4 weeks for clinical effects to manifest due to hormone store depletion.
Adverse effects of thioamides occur in 3-12% of patients, with common reactions including rash and fever, and rare effects like agranulocytosis and hepatitis.
Anion inhibitors (e.g., perchlorate, SCN^-, pertechnetate) block iodide uptake but can be reversed by high iodide doses.
Iodides inhibit hormone release and decrease thyroid size; however, they may induce hyperthyroidism in susceptible individuals and are contraindicated in pregnancy.

Beta Blockers

Beta blockers like propranolol are effective for managing symptoms of thyrotoxicosis, as symptoms mirror sympathetic stimulation.
Non-selective beta blockers are preferred, especially in patients resistant to other therapies.

Special Conditions

Thyroid Storm

A life-threatening thyrotoxic crisis characterized by an acute exacerbation of thyrotoxicosis symptoms.
Treatment includes propranolol, calcium channel blockers, potassium iodide, thioamides, and corticosteroids to mitigate effects.

Thyrotoxicosis in Pregnancy

Recommended treatment involves subtotal thyroidectomy and I^131^, with operative management necessary during the mid-trimester, supplemented post-surgery with thyroid hormones.

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Description

Explore the complex functions of thyroid hormones and the role of anti-thyroid agents in managing thyroid disorders. This quiz delves into the characteristics of the thyroid gland, including hormone production and the implications of goiter. Test your understanding of key concepts related to thyroxine, triiodothyronine, and calcitonin.