Thyroid Hormones and Metabolism Quiz
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Questions and Answers

What percentage of T4 in blood is unbound or free?

  • 0.04%
  • 0.1%
  • 0.02% (correct)
  • 0.4%

Which of the following serum proteins binds the highest percentage of T4?

  • Thyroid binding globulin (TBG) (correct)
  • Transferrin
  • Thyroid binding prealbumin (TBPA)
  • Albumin

What is one of the main functions of the nervous system mentioned?

  • Stores energy
  • Controls muscle contractions
  • Regulates digestion
  • Enhances wakefulness and alertness (correct)

What happens to T4 during its metabolism?

<p>It is converted to T3 and reverse T3 (B)</p> Signup and view all the answers

Which thyroid hormone does reverse T3 block the action of?

<p>T3 (B)</p> Signup and view all the answers

What class do individuals with normal healthy levels of thyroid hormones belong to?

<p>Euthyroid (B)</p> Signup and view all the answers

How is some T4 and T3 excreted from the body?

<p>Excreted in bile and urine (B)</p> Signup and view all the answers

What is the primary purpose of thyroid tests mentioned?

<p>Grouping individuals based on thyroid hormone levels (A)</p> Signup and view all the answers

What happens to TSH levels after a TRH injection in cases of Primary Hypothyroidism?

<p>They increase and take longer to return to basal levels. (C)</p> Signup and view all the answers

What is the main value of the TRH test?

<p>Diagnosis of hyperthyroidism. (D)</p> Signup and view all the answers

Which type of thyroid auto-antibody is present in over 80% of patients with Hashimoto’s disease?

<p>Complement-fixing antibodies. (B)</p> Signup and view all the answers

What is the role of thyroid-stimulating immunoglobulins (TSI) in Graves’ disease?

<p>They bind to and activate the TSH receptor. (B)</p> Signup and view all the answers

Which antibodies are detected in all forms of autoimmune thyroiditis?

<p>Colloid antigen antibodies. (A)</p> Signup and view all the answers

What will the radioactive uptake tests measure?

<p>Uptake of iodine and technetium by the thyroid gland. (C)</p> Signup and view all the answers

In hyperthyroidism, what is the rate of uptake of $^{132}I$ by the thyroid typically?

<p>Increased in over 90% of cases. (B)</p> Signup and view all the answers

What two hormones are predominantly secreted by the thyroid gland?

<p>Triiodothyronine and Thyroxine (C)</p> Signup and view all the answers

What is the half-life of technetium 99m ($^{99m}Tc$)?

<p>6 hours. (D)</p> Signup and view all the answers

Which of the following statements about T3 and T4 is correct?

<p>T3 contains three iodine atoms and T4 contains four iodine atoms. (B)</p> Signup and view all the answers

What is the primary structural characteristic of the thyroid gland?

<p>It consists of two lobes connected by an isthmus. (D)</p> Signup and view all the answers

What role does iodine play in thyroid hormones?

<p>It is integral to the structure of thyroid hormones. (B)</p> Signup and view all the answers

Which organ is NOT affected by thyroid hormones?

<p>Spleen (B)</p> Signup and view all the answers

Which of the following is true regarding thyroid vesicles?

<p>They contain a colloid substance with thyroglobulin. (D)</p> Signup and view all the answers

Which hormone from the thyroid gland has no biological activity?

<p>Reverse T3 (A)</p> Signup and view all the answers

What is the normal weight of a healthy thyroid gland?

<p>28 g (A)</p> Signup and view all the answers

What is the primary function of the thyroid gland?

<p>To concentrate and trap iodide for hormone production (A)</p> Signup and view all the answers

Which hormone controls the secretion of thyroid hormones?

<p>Thyroid-stimulating hormone (TSH) (C)</p> Signup and view all the answers

What percentage of the body's iodine is concentrated in the thyroid gland?

<p>25% (D)</p> Signup and view all the answers

What biological activity does T4 possess?

<p>T4 is entirely inactive biologically (A)</p> Signup and view all the answers

How does T3 primarily get produced in the body?

<p>It is derived from T4 by removing an iodine atom in the tissues (B)</p> Signup and view all the answers

Which of the following is NOT a role of thyroid hormones?

<p>Controlling blood sugar levels (A)</p> Signup and view all the answers

What effect do T3 and T4 have on the secretion of TSH?

<p>They have a negative feedback effect (A)</p> Signup and view all the answers

Which of the following correctly describes thyroglobulin?

<p>It is rich in iodine and involved in hormone production (C)</p> Signup and view all the answers

What is a characteristic of hypothyroidism in relation to radioactive iodine uptake?

<p>It shows reduced uptake of 132I. (B)</p> Signup and view all the answers

Which of the following is NOT a disadvantage of in vivo radioactive uptake tests?

<p>Requires extensive patient preparation. (D)</p> Signup and view all the answers

What condition is associated with cold nodules that may be solid or cystic?

<p>Hypofunctioning nodules. (B)</p> Signup and view all the answers

What is one of the tests affected by thyroid gland activity?

<p>Basal Metabolic Rate (BMR). (B)</p> Signup and view all the answers

What is the potential consequence of untreated congenital hypothyroidism during the critical first two years of life?

<p>Permanent brain damage. (D)</p> Signup and view all the answers

What symptom is associated with a typical case of congenital hypothyroidism?

<p>Thick, dry skin. (C)</p> Signup and view all the answers

In relation to plasma LDL-cholesterol levels, which condition often shows markedly increased levels?

<p>Hypothyroidism. (D)</p> Signup and view all the answers

What happens to growth if treatment for congenital hypothyroidism is initiated while the epiphyses are undamaged?

<p>Growth will resume. (B)</p> Signup and view all the answers

What is the typical plasma T4 level in genuine Hypothyroidism?

<p>Less than 70 nmol/L (B)</p> Signup and view all the answers

What percentage of Total T3 is considered Free T3, which is available for biological activity?

<p>0.3% (B)</p> Signup and view all the answers

In cases of T3 Toxicosis, what usually differentiates it from other forms of Hyperthyroidism?

<p>Total T4 and Free T4 are both normal while Total T3 and Free T3 are raised (C)</p> Signup and view all the answers

Why is Total T4 less sensitive than TSH levels for diagnosing hypothyroidism?

<p>Total T4 is affected by binding proteins (A)</p> Signup and view all the answers

What happens to Total T3 concentrations with alterations in binding proteins?

<p>They change but to a lesser degree than T4 (B)</p> Signup and view all the answers

What is the primary utility of measuring Total T3 in clinical practice?

<p>To diagnose T3-toxicosis (C)</p> Signup and view all the answers

What is Free Thyroxine Index (FTI) used for?

<p>To estimate plasma FT4 without direct measurement (A)</p> Signup and view all the answers

Which condition can lead to low Total T3 levels in the absence of thyroid disease?

<p>Chronic illness or myocardial infarction (B)</p> Signup and view all the answers

Flashcards

Thyroid gland function

The thyroid gland concentrates iodide, produces and stores thyroid hormones. These hormones regulate metabolic functions.

Thyroglobulin

A protein in the thyroid gland that is rich in iodine. It is essential for the production of thyroid hormones.

Iodine in the body

The thyroid gland stores 25% of the body's iodine, which is obtained from diet. Iodine is concentrated in the thyroid to create thyroid hormones.

Thyroid hormones & metabolism

Thyroid hormones influence many metabolic functions, including growth, mental development, and sexual maturation.

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Thyroid hormone regulation (TSH)

The pituitary gland releases thyroid-stimulating hormone (TSH), which controls how much thyroid hormone the thyroid gland produces.

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Negative feedback system

Thyroid hormones (T3 and T4) have a negative feedback effect on the pituitary and hypothalamus, regulating their production of TSH and TRH.

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T3 vs. T4

T3 is the active form of thyroid hormone, while T4 is converted into T3. T3 is more potent in its actions.

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Thyroid hormone effects

Thyroid hormones increase protein synthesis, enhance mitochondrial respiration, and are essential for normal growth.

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Thyroid Gland

An endocrine gland located in front of and on each side of the larynx. It secretes triiodothyronine (T3) and thyroxine (T4) which control metabolism and growth.

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Thyroid Hormones

Triiodothyronine (T3) and thyroxine (T4) are hormones produced by the thyroid gland, responsible for regulating metabolism and growth.

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Thyroid Vesicles

Small sacs within the thyroid gland, formed by cuboidal epithelial cells and filled with colloid containing thyroglobulin, T3, and T4.

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Iodine's Role in Thyroid Hormones

Iodine is a crucial component of thyroid hormones. It is found in significant proportions, hence the names triiodothyronine and tetraiodothyronine, referring to the number of iodine atoms.

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Reverse T3

A form of T3 that lacks biological activity.

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Thyroid Hormone Function

Thyroid hormones regulate metabolism by influencing energy production, growth, and development.

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Iodine Deficiency Effects

Too much or too little iodine can cause detrimental issues as it is essential for thyroid hormone synthesis.

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Thyroid Hormone Binding

Most T4 in the blood is bound to proteins like thyroid binding globulin (TBG), thyroid binding prealbumin (TBPA), and albumin. Only a tiny fraction of T4 is free and active.

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Free T4 and T3

Only a small percentage of total T4 and T3 is unbound, known as 'free' T4 and T3. This free form is biologically active and can interact with target cells.

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Reverse T3 (rT3)

A less active form of T3 produced by the body. Unlike T3, rT3 cannot stimulate thyroid hormone receptors, but it can block the action of T3.

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T4 to T3 Conversion

T4 is converted to T3, the more active form, in the liver and various other tissues. This conversion happens through deiodination.

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T4 and T3 Metabolism

T4 and T3 are mostly broken down in the liver. They are conjugated with glucuronide or sulfate, excreted in bile, and partially hydrolyzed in the intestines.

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Thyroid Hormone Excretion

A small amount of T4 and T3 is excreted in urine, while most is metabolized and recycled. Conjugated T4 and T3 are released in bile, possibly reabsorbed in the intestines.

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Euthyroid

A state of normal thyroid hormone levels. The thyroid gland is functioning correctly and producing appropriate amounts of thyroid hormones.

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Hyperthyroid vs. Hypothyroid

Hyperthyroidism is when the thyroid produces too much hormone, leading to increased metabolism. Hypothyroidism is when the thyroid produces too little hormone, leading to decreased metabolism.

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Free T4 Levels

The amount of unbound, biologically active thyroid hormone (T4) in the blood. These levels remain constant despite variations in thyroid hormone binding proteins.

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T3 Toxicosis

A type of hyperthyroidism where the thyroid gland produces excess T3, leading to symptoms even if T4 levels are normal.

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Free T4 vs Total T4

Free T4 is the unbound, active form of T4, while Total T4 includes both the free and bound forms. Free T4 is a better indicator of thyroid function.

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Free Thyroid Hormones

Unbound thyroid hormones (FT4 and FT3) that are biologically active and can directly interact with cells.

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Free Thyroxine Index (FTI)

A calculation that estimates the level of free T4 in the blood without directly measuring it.

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Why is Free T4 a better test?

Free T4 accurately reflects the amount of active thyroid hormone in the body, unlike total T4 that can be influenced by protein binding.

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How does Free T3 diagnose T3-toxicosis?

Free T3 levels are unaffected by changes in protein levels, making it a reliable test in cases where total T3 levels might be misleading.

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TRH test in Hypothyroidism

In Primary Hypothyroidism, TSH levels are high at baseline and rise even further after injecting TRH. They also take longer than usual to return to normal.

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TRH test for Hyperthyroidism

The TRH test is primarily used to diagnose Hyperthyroidism. If TSH levels increase after TRH injection, it excludes Hyperthyroidism.

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Thyroid Autoantibodies: What are they?

Antibodies targeting thyroid tissue are found in the blood of patients with thyroid diseases. They indicate an autoimmune condition.

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Complement-fixing antibodies

These antibodies are specific for thyroid tissue and found in high percentages of patients with Hashimoto's disease.

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Thyroglobulin antibodies

Found in most cases of early hypothyroidism and in a large proportion of hyperthyroid patients.

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Colloid antigen antibodies

Found in all forms of autoimmune thyroiditis. They aid in diagnosing De Quervain's thyroiditis.

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Thyroid-stimulating immunoglobulins (TSI)

IgG antibodies that target TSH receptors in the thyroid. Found in patients with Graves' disease. These antibodies stimulate thyroid hormone production.

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Thyroid Growth Immunoglobulins (TGI)

These antibodies induce thyroid growth but do not stimulate hormone production. Found in patients suffering from Graves' disease.

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Hypothyroid Iodine Uptake

In hypothyroidism, the thyroid gland's ability to absorb iodine (132I) is significantly reduced and occurs much slower than in a healthy person.

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In Vivo Radioactive Uptake Test Disadvantages

These tests have limitations, including difficulty in interpreting results and being unsuitable for pregnant women.

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Hot Thyroid Nodule

A thyroid nodule that is hyperactive, meaning it takes up more iodine and produces more hormones. Typically benign.

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Cold Thyroid Nodule

A thyroid nodule that is less active, meaning it takes up less iodine and produces fewer hormones. May be solid or cystic. Some may be cancerous.

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Basal Metabolic Rate (BMR)

A measure of how much energy your body uses at rest. It is raised in hyperthyroidism and reduced in hypothyroidism.

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Glucose Tolerance Test in Hyperthyroidism

Sometimes abnormal in hyperthyroid patients. They may exhibit a diabetic-like response to glucose.

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Plasma LDL Cholesterol in Hypothyroidism

Often significantly elevated in hypothyroidism. It's a 'bad' cholesterol linked to heart disease.

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Congenital Hypothyroidism

A condition present at birth where the thyroid gland doesn't produce enough hormones. It can lead to severe developmental problems.

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Study Notes

Thyroid Metabolism and Function Tests

  • The thyroid gland is an endocrine gland located in front of and on each side of the larynx.
  • It secretes two non-steroid hormones: triiodothyronine (T3) and thyroxine (T4).
  • These hormones control metabolism and growth.
  • The thyroid gland is a brownish-red organ with two lobes connected by an isthmus.
  • It typically weighs approximately 28 grams.
  • The gland is composed of cuboidal epithelial cells arranged in small sacs called vesicles or follicles.
  • Follicles are supported by connective tissue forming a framework for the entire gland.
  • In a normal thyroid gland, vesicles are usually filled with colloid which contains thyroglobulin, combined with T3 and T4.
  • T4 (thyroxine) is also called tetraiodothyronine.
  • T3 (triiodothyronine) is another important thyroid hormone.
  • These hormones are made of the amino acid tyrosine.
  • T3 contains three iodine atoms, and T4 contains four iodine atoms.

Thyroid Hormones in the Blood

  • Approximately 99.98% of T4 is bound to three serum proteins: Thyroid binding globulin (TBG), Thyroid binding prealbumin (TBPA), and Albumin.
  • Only about 0.02% of the total T4 in the blood is unbound (free).
  • Only about.04% of total T3 is free.
  • The main function of these proteins is to bind and transport thyroid hormones through the bloodstream.

Thyroid Hormone Effects on the Body

  • Thyroid hormones regulate and speed up metabolic functions in the body, influencing:
    • Normal growth
    • Mental development
    • Sexual maturation
    • Cardiac output
    • Heart rate
    • Cellular protein synthesis
    • Mitochondrial respiration (increasing oxygen consumption)

Thyroid Hormone Regulation

  • The hypothalamus and pituitary gland control the normal secretion of thyroid hormones.
  • Thyroid-releasing hormone (TRH) is secreted by the hypothalamus.
  • TRH stimulates the anterior pituitary to release thyroid-stimulating hormone (TSH).
  • TSH stimulates the synthesis and secretion of T3 and T4 by the thyroid gland.
  • T4 and T3 have a negative feedback on the pituitary and the hypothalamus, regulating TSH output.

Thyroid Hormone Metabolism - T4 to T3

  • Most of the T3 in the plasma is derived from T4 by the removal of an iodine atom in the tissues.
  • T3 is much more chemically active than T4, responsible for the majority of biological effects.
  • Deamination and deiodination are processes related to the further metabolism of T4 to T3 and reverse T3.
  • Following deionization and deamination, thyroid metabolites are conjugates, which are excreted in the bile.

Types of Thyroid Disorders

  • Hyperthyroidism: characterized by high levels of thyroid hormones, often due to an overactive thyroid.
    • Symptoms include intolerance to heat, fine straight hair, bulging eyes and facial flushing.
    • Diagnosis can be ruled out by increased TSH plasma response following TRH injection.
    • Diagnoses frequently include TBG excess or pregnancy conditions.
  • Hypothyroidism: characterized by low levels of thyroid hormones.
    • Symptoms include intolerance to cold, receding hairline, facial and eyelid edema, thick tongue, and slow speech.
    • Diagnoses frequently include conditions involving the immune system, damage to the pituitary. or a genetic deficiency.

Tests for Thyroid Function

  • Tests to evaluate thyroid function include:
    • Total Thyroxine (T4), Total Tri-iodothyronine (T3), Free Thyroxine (FT4), Free T3 (FT3), Free Thyroxine Index (FTI).
    • In vivo radioactive iodine (132 I) or technetium 99m (99mTc) uptake by the thyroid gland.
    • Basal Metabolic Rate (BMR).
    • Glucose tolerance test.
    • Plasma calcium.
    • Plasma LDL-cholesterol.
    • Thyroid autoantibodies to assess autoimmune disorders.
  • Measurement of TSH, thyroid releasing hormone (TRH) to identify hypothalamic-pituitary-thyroid (HPT) axis problems.
  • Reference ranges for these tests help in diagnosing hypothyroidism or hyperthyroidism.

Thyroid Cases

  • Congenital hypothyroidism: Thyroid hormone is necessary for brain development during the first two years of life. A deficiency can result in permanent brain damage.
    • Screening infants at birth is vital for early detection.
  • Primary hypothyroidism: Thyroid gland dysfunction preventing proper thyroid hormone production.
  • Apparent hypothyroidism: False-positive hypothyroidism caused by decreased TBG.

Additional Notes

  • Thyroid hormones are unique for their iodine content (59-65%).
  • Iodine is integral to the hormone's structure and the high proportion of iodine is responsible for possible detrimental effects.
  • Various factors can influence the level of thyroid hormones in blood, including pregnancy or steroid use, which can be apparent hypothyroidism/hyperthyroidism.

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Thyroid Metabolism and Function Tests (PDF)

Description

Test your knowledge on thyroid hormones with this quiz covering topics such as T4 binding, serum proteins, and the nervous system's functions. Explore how T4 is metabolized and its significance in thyroid tests. Ideal for students studying endocrinology or related fields.

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