Endocrinology Mechanisms of Hypothyroidism

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Questions and Answers

What is the main result of hypothyroidism in the body?

Increased metabolic rate

Slowing down of all body functions (correct)

Improved cognitive function

Enhanced physical growth

Which condition is associated with growth and developmental retardation in infants and children due to hypothyroidism?

Hyperthyroidism

Goiter

Dwarfism

Cretinism (correct)

What laboratory finding is typical in the diagnosis of hypothyroidism?

Low levels of free T4 and elevated TSH (correct)

Normal free T4 and low TSH

High free T4 with normal TSH

High levels of free T4 and TSH

Which of the following is not a cause of hypothyroidism?

Excess iodine intake Signup and view all the answers

What is the treatment for drug-induced hypothyroidism?

Withdrawal of the drug and levothyroxine replacement Signup and view all the answers

In infants, what is the recommended daily dosage of levothyroxine for hypothyroidism treatment?

10-15 µg/kg/day Signup and view all the answers

Which of the following statements about Hashimoto's thyroiditis is true?

It is an autoimmune disorder causing destruction of the thyroid. Signup and view all the answers

What differentiates dyshormoogenesis from other causes of hypothyroidism?

Impaired synthesis of T4 due to enzyme deficiency Signup and view all the answers

Which skin condition is associated with hyperthyroidism?

Warm moist skin Signup and view all the answers

What metabolic change is observed in hypothyroidism?

Decreased basal metabolic rate Signup and view all the answers

What reproductive effect is commonly seen in hypothyroidism?

Decreased fertility Signup and view all the answers

Which of the following is a haematopoietic effect of hyperthyroidism?

Increased erythropoiesis Signup and view all the answers

What renal effect is observed in hyperthyroidism?

Increased renal blood flow Signup and view all the answers

What is a common musculoskeletal symptom of hypothyroidism?

Weakness and muscle fatigue Signup and view all the answers

Which neurological symptom is linked with hyperthyroidism?

Hyperkinesia Signup and view all the answers

What gastrointestinal change is associated with hypothyroidism?

Decreased frequency of bowel movement Signup and view all the answers

What metabolic change occurs in hyperthyroidism concerning insulin?

Delayed degradation of insulin Signup and view all the answers

Which of the following describes the skin condition associated with hypothyroidism?

Cool and puffy skin Signup and view all the answers

What is the primary mechanism of action for Thioamides?

Preventing hormone synthesis by inhibiting thyroid peroxidase Signup and view all the answers

Which Thioamide is about ten times more effective than propylthiouracil?

Carbamizole Signup and view all the answers

What percentage of Methimazole is excreted via the kidneys in the first 48 hours?

65-70% Signup and view all the answers

Which drug has a bioavailability of about 50-80% and a short half-life of approximately 1.5 hours?

Propylthiouracil Signup and view all the answers

What is the average daily dose for Methimazole?

30 mg Signup and view all the answers

Why is Propylthiouracil preferred during pregnancy?

It crosses the placenta less readily Signup and view all the answers

What happens to Methimazole and Carbamizole when administered?

Both drugs accumulate in the thyroid gland Signup and view all the answers

What occurs to the half-life of Methimazole despite its short duration?

It remains the same due to accumulation in the thyroid Signup and view all the answers

What is the primary mechanism of action for T4 and T3 in the cells?

They bind to specific receptors and activate RNA formation. Signup and view all the answers

Which tissues have a large number of T3 receptors?

Liver and kidney Signup and view all the answers

How does T4 compare in affinity to the T3 receptor?

T4 shows about 10 times lower affinity than T3. Signup and view all the answers

Which form of thyroxine is associated with activation in the body?

T3 Signup and view all the answers

What role do specific T3 receptor proteins play in the body?

They activate receptors leading to increased protein synthesis. Signup and view all the answers

Where is reverse T3 primarily found in the body?

It is mostly inactive. Signup and view all the answers

What is a consequence of using the radioactive iodine treatment in hyperthyroidism for patients over 40 years old?

Higher risk of genetic damage Signup and view all the answers

What is the primary role of beta blockers in the treatment of thyrotoxicosis?

To block the symptoms of sympathetic stimulation Signup and view all the answers

What is the primary treatment for a thyroid storm?

Administration of beta blockers like Propanolol Signup and view all the answers

Which agent is commonly used for thyroid hormone synthesis blockade?

Potassium iodide Signup and view all the answers

During pregnancy, which treatment approach is recommended for women with thyrotoxicosis?

Surgical removal of part of the gland with Iodine I^131^ Signup and view all the answers

What is the recommended dosage for Propanolol during a thyroid storm?

1-2 mg slow IV initially, followed by 40-80 mg orally every 6 hours Signup and view all the answers

What is a critical component of supportive therapy in managing thyrotoxicosis?

Close monitoring and hydration Signup and view all the answers

Which of the following is NOT a symptom of thyrotoxicosis?

Sudden weight gain Signup and view all the answers

Study Notes

Mechanism of Action of Thyroid Hormones

T4 and T3 dissociate from thyroglobulin proteins and enter cells by diffusion or active transport.
T4 binds to specific T3 receptor proteins in the nucleus, with alpha and beta forms present.
Major sites of T3 receptors include the liver, kidney, heart, lungs, skeletal muscles, pituitary, and intestines, with lesser amounts in the brain, spleen, and testes.
Binding activates receptors, increasing RNA formation and protein synthesis, leading to physiological responses.
T4 has about 10 times lower affinity for T3 receptors compared to T3.

Chemistry of Thyroid Hormones

T4 (Thyroxine) is converted to T3 (Triiodothyronine) through deiodination for activation.
Reverse T3, an inactive form, is also produced.
All forms of thyroid hormones are levo-isomers.

Pharmacological Effects of Thyroid Hormones

Thyrotoxicosis:
- Warm, moist skin; heat intolerance; excessive sweating; thin hair; periorbital dermopathy.
- Increased basal metabolic rate; hyperglycemia; elevated triglycerides and cholesterol; weight loss.
- Menstrual irregularities and increased gonad steroid metabolism.
- Increased erythropoiesis and possible anemia.
- Mild polyuria; increased renal blood flow; increased GFR.
- Nervousness; hyperkinesia; emotional liability; insomnia.
- Increased appetite; frequent bowel movements.
Hypothyroidism:
- Pale, cool, puffy skin; dry, brittle hair; brittle nails.
- Decreased basal metabolic rate; delayed insulin degradation; reduced triglycerides and cholesterol.
- Infertility, decreased libido, and menstrual irregularities.
- Decreased erythropoiesis; possible anemia.
- Decreased renal blood flow; reduced GFR; impaired water excretion.
- Stiffness, muscle fatigue; decreased deep tendon reflexes.
- Lethargy; general slowing of mental processes.
- Decreased appetite; less frequent bowel movements.

Hypothyroidism Overview

Syndrome resulting from a deficiency of thyroid hormones, reversible but leads to slowing of body functions.
In infants, can cause growth and developmental retardation, possibly resulting in dwarfism and irreversible mental retardation.
Diagnosis involves low free T4 and elevated serum TSH levels.

Causes of Hypothyroidism

Drug-Induced: Medications like lithium, fluoride, thioamide blockers, etc., lead to mild to moderate hypothyroidism.
Dyshormogenesis: Impaired synthesis of T4 due to enzyme deficiencies; may present with goiter.
Destruction/Removal: Radiation, thyroidectomy leads to severe hypothyroidism without goiter.
Congenital: Cretinism due to iodine deficiency; severe hypothyroidism results.
TSH Absence: Pituitary/hypothalamic disorders lead to mild hypothyroidism.
Hashimoto's Thyroiditis: Autoimmune destruction of the thyroid; progression from goiter to mild/moderate hypothyroidism is common.

Management of Hypothyroidism

Withdrawal of drugs causing hypothyroidism, with short-term levothyroxine replacement therapy.
Levothyroxine dosage for infants is typically 10-15 µg/kg/day, while adults require about 1.7 µg/kg/day.
Other management techniques involve modifications of tissue response to hormones and surgical interventions.

Thioamides

Include Methimazole, Carbamizole, and Propylthiouracil; Carbamizole is ten times more effective than Propylthiouracil.
Mechanism: They prevent hormone synthesis by inhibiting thyroid peroxidase and block iodine organification.
Side effects may include radiation-induced genetic damage; caution is advised for patients over 40.

Beta Blockers

Useful for symptom control in thyrotoxicosis as they counteract sympathetic stimulation.
Propranolol is the most commonly used beta blocker for managing symptoms.

Thyroid Storm

Acute exacerbation of thyrotoxicosis symptoms; life-threatening condition.
Treatment includes Propranolol, Diltiazem, potassium iodide, iodinated contrast agents, and corticosteroids.

Thyrotoxicosis in Pregnancy

Recommended treatment includes subtotal thyroidectomy and Iodine I^131.
Propylthiouracil is used during pregnancy, and thyroid supplements may be necessary post-surgery.

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Description

This quiz explores the mechanisms of action related to hypothyroidism, focusing on how T4 and T3 dissociate from thyroglobulin proteins and enter cells. It covers the binding of T3 to receptor proteins and their distribution across various organs. Test your understanding of these key physiological processes in endocrinology.