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?

It is converted to T3 and reverse T3 Signup and view all the answers

Which thyroid hormone does reverse T3 block the action of?

T3 Signup and view all the answers

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

Euthyroid Signup and view all the answers

How is some T4 and T3 excreted from the body?

Excreted in bile and urine Signup and view all the answers

What is the primary purpose of thyroid tests mentioned?

Grouping individuals based on thyroid hormone levels Signup and view all the answers

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

They increase and take longer to return to basal levels. Signup and view all the answers

What is the main value of the TRH test?

Diagnosis of hyperthyroidism. Signup and view all the answers

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

Complement-fixing antibodies. Signup and view all the answers

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

They bind to and activate the TSH receptor. Signup and view all the answers

Which antibodies are detected in all forms of autoimmune thyroiditis?

Colloid antigen antibodies. Signup and view all the answers

What will the radioactive uptake tests measure?

Uptake of iodine and technetium by the thyroid gland. Signup and view all the answers

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

Increased in over 90% of cases. Signup and view all the answers

What two hormones are predominantly secreted by the thyroid gland?

Triiodothyronine and Thyroxine Signup and view all the answers

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

6 hours. Signup and view all the answers

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

T3 contains three iodine atoms and T4 contains four iodine atoms. Signup and view all the answers

What is the primary structural characteristic of the thyroid gland?

It consists of two lobes connected by an isthmus. Signup and view all the answers

What role does iodine play in thyroid hormones?

It is integral to the structure of thyroid hormones. Signup and view all the answers

Which organ is NOT affected by thyroid hormones?

Spleen Signup and view all the answers

Which of the following is true regarding thyroid vesicles?

They contain a colloid substance with thyroglobulin. Signup and view all the answers

Which hormone from the thyroid gland has no biological activity?

Reverse T3 Signup and view all the answers

What is the normal weight of a healthy thyroid gland?

28 g Signup and view all the answers

What is the primary function of the thyroid gland?

To concentrate and trap iodide for hormone production Signup and view all the answers

Which hormone controls the secretion of thyroid hormones?

Thyroid-stimulating hormone (TSH) Signup and view all the answers

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

25% Signup and view all the answers

What biological activity does T4 possess?

T4 is entirely inactive biologically Signup and view all the answers

How does T3 primarily get produced in the body?

It is derived from T4 by removing an iodine atom in the tissues Signup and view all the answers

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

Controlling blood sugar levels Signup and view all the answers

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

They have a negative feedback effect Signup and view all the answers

Which of the following correctly describes thyroglobulin?

It is rich in iodine and involved in hormone production Signup and view all the answers

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

It shows reduced uptake of 132I. Signup and view all the answers

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

Requires extensive patient preparation. Signup and view all the answers

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

Hypofunctioning nodules. Signup and view all the answers

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

Basal Metabolic Rate (BMR). Signup and view all the answers

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

Permanent brain damage. Signup and view all the answers

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

Thick, dry skin. Signup and view all the answers

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

Hypothyroidism. Signup and view all the answers

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

Growth will resume. Signup and view all the answers

What is the typical plasma T4 level in genuine Hypothyroidism?

Less than 70 nmol/L Signup and view all the answers

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

0.3% Signup and view all the answers

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

Total T4 and Free T4 are both normal while Total T3 and Free T3 are raised Signup and view all the answers

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

Total T4 is affected by binding proteins Signup and view all the answers

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

They change but to a lesser degree than T4 Signup and view all the answers

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

To diagnose T3-toxicosis Signup and view all the answers

What is Free Thyroxine Index (FTI) used for?

To estimate plasma FT4 without direct measurement Signup and view all the answers

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

Chronic illness or myocardial infarction Signup and view all the answers

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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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.