Thyroid Gland Overview and Functions

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Questions and Answers

What is the effect of increased thyroid hormone on plasma fat concentrations?

Decreases triglycerides levels (correct)

Increases phospholipid levels

Maintains triglycerides levels

Increases cholesterol levels

What happens to plasma cholesterol levels during prolonged hypothyroidism?

Cholesterol levels increase drastically (correct)

Cholesterol levels remain unchanged

Cholesterol levels fluctuate widely

Cholesterol levels decrease significantly

How does thyroid hormone affect liver cells in relation to low-density lipoproteins?

Decreases the number of LDL receptors

Increases the number of LDL receptors (correct)

Alters the LDL receptor function

Has no effect on LDL receptors

What is a consequence of excess thyroid hormone on vitamin requirements?

Causes increased need for vitamins Signup and view all the answers

What is the potential increase in basal metabolic rate due to excessive thyroid hormone?

60 to 100 percent Signup and view all the answers

What effect does the absence of thyroid hormone have on the basal metabolic rate?

Decreases it to nearly half of normal Signup and view all the answers

What is a common result of greatly increased thyroid hormone levels?

Decreased body weight Signup and view all the answers

Which of the following mechanisms helps lower plasma cholesterol concentrations due to thyroid hormone?

Increased cholesterol secretion in bile Signup and view all the answers

What is a common cause of the enlarged thyroid gland in idiopathic nontoxic colloid goiter?

Mild thyroiditis Signup and view all the answers

Which enzyme deficiency is associated with the inability to recover iodine from iodinated tyrosines?

Deiodinase enzyme Signup and view all the answers

How do goitrogenic substances contribute to thyroid enlargement?

By promoting TSH-stimulated enlargement Signup and view all the answers

Which of the following is NOT one of the enzyme deficiencies related to abnormal thyroid hormone formation?

Amino acid synthesis enzyme Signup and view all the answers

What is the typical secretion pattern of thyroid hormones in patients with idiopathic colloid goiter?

Depressed hormone secretion Signup and view all the answers

What is the required amount of ingested iodine for the normal formation of thyroxine each year?

50 milligrams Signup and view all the answers

What is the purpose of iodizing common table salt?

To prevent iodine deficiency Signup and view all the answers

How much concentration does the iodide pump normally achieve in the thyroid gland compared to blood concentration?

Which factor most significantly influences the rate of iodide trapping by the thyroid?

Concentration of TSH Signup and view all the answers

What is the molecular weight of thyroglobulin?

335,000 Signup and view all the answers

What portion of iodides ingested orally is typically removed by the thyroid gland for hormone synthesis?

One-fifth Signup and view all the answers

What mechanism describes the transport of iodides from the blood into thyroid glandular cells?

Iodide trapping Signup and view all the answers

What occurs to the activity of the iodide pump in thyroid cells after hypophysectomy?

It greatly diminishes Signup and view all the answers

What is the primary substance that thyroxine is converted to in the body?

Triiodothyronine Signup and view all the answers

How do thyroid hormone receptors typically interact with DNA?

They form heterodimers with retinoid X receptors. Signup and view all the answers

What is a key outcome of thyroid hormone action on the cell?

Initiation of RNA translation Signup and view all the answers

What effect do thyroid hormones have on the basal metabolic rate?

It can increase to 60% to 100% above normal. Signup and view all the answers

What occurs to the mitochondria in response to thyroxine or triiodothyronine administration?

They increase in size and number. Signup and view all the answers

What dual effect do thyroid hormones have on protein synthesis and catabolism?

Increase in both synthesis and catabolism. Signup and view all the answers

Which of the following effects is NOT associated with an increase in thyroid hormones?

Inhibition of other endocrine gland activities. Signup and view all the answers

What percentage of thyroid hormone molecules that bind with receptors is typically triiodothyronine?

More than 90% Signup and view all the answers

What is the primary function of triiodothyronine (T3) compared to thyroxine (T4)?

T3 acts more rapidly and intensively than T4. Signup and view all the answers

Which hormone primarily regulates thyroid secretion?

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

What happens to the basal metabolic rate in the absence of thyroid secretion?

It decreases by 40 to 50% below normal. Signup and view all the answers

What is the main component of the colloid in the thyroid gland?

Thyroglobulin Signup and view all the answers

How much of the thyroid gland's secreted hormones is typically thyroxine?

93% Signup and view all the answers

What structural feature characterizes the thyroid gland?

Closed follicles filled with colloid. Signup and view all the answers

What is the approximate weight range of the thyroid gland in adults?

15 to 20 grams Signup and view all the answers

Which of the following statements is true regarding the conversion of thyroxine to triiodothyronine?

Almost all thyroxine is eventually converted to triiodothyronine in the tissues. Signup and view all the answers

Study Notes

Overview of the Thyroid Gland

Located below the larynx, anterior to the trachea, the thyroid gland is one of the largest endocrine glands, weighing 15 to 20 grams in adults.
Secretes two main hormones: thyroxine (T4) and triiodothyronine (T3), which significantly increase metabolic rates.

Hormonal Functions and Effects

Complete lack of thyroid secretion can lower the basal metabolic rate by 40-50%, while excess secretion can elevate it by 60-100%.
Thyroxine accounts for about 93% of thyroid hormones, and triiodothyronine makes up 7%, with T3 being four times more potent than T4.
Thyroid hormones enhance metabolic activity across almost all body tissues, stimulating food utilization for energy and increasing both protein synthesis and catabolism.

Structure and Composition of the Thyroid Gland

The thyroid consists of numerous follicles filled with colloid and lined with cuboidal epithelial cells that secrete hormones.
Colloid contains thyroglobulin, a large glycoprotein that stores thyroid hormones.
High blood flow to the thyroid is noted, approximately five times its weight per minute.

Iodine Requirement and Hormone Formation

Each year, around 50 mg of iodine is necessary for adequate thyroxine production (about 1 mg per week).
Iodized salt is commonly used to prevent iodine deficiency, with a ratio of 1 part sodium iodide to 100,000 parts sodium chloride.

Iodide Handling and Absorption

Iodides ingested are absorbed from the gastrointestinal tract, with most rapidly excreted but about one-fifth retained by the thyroid.
"Iodide trapping" actively transports iodides into thyroid cells, concentrating them up to 30 times the blood level, potentially 250 times during max activity.

Synthesis of Hormones

Thyroglobulin is synthesized by thyroid cells and stored in follicles for hormone production.
Triiodothyronine is formed by the removal of one iodide from most thyroxine molecules.
Intracellular hormone receptors have a strong affinity for T3, with over 90% of binding occurring with T3.

Regulatory Mechanisms

TSH (thyroid-stimulating hormone) from the anterior pituitary gland primarily regulates thyroid hormone secretion.
Thyroid receptors, upon hormone binding, activate gene transcription, leading to the formation of various proteins that influence metabolic activities.

Metabolic Effects of Thyroid Hormones

Increase in basal metabolic rate due to heightened metabolic activities; can lead to weight loss or maintenance.
Enhances mitochondrial size and number, boosting cellular energy production.

Lipid Metabolism

Increased thyroid hormone levels reduce plasma cholesterol, triglycerides, and phospholipids while raising free fatty acids.
Low thyroid hormone levels can raise cholesterol levels and promote fat deposition in the liver.

Vitamin Requirements

Elevated thyroid hormone secretion raises demand for vitamins due to increased enzyme production.
A relative vitamin deficiency can occur if vitamin intake does not match increased requirements.

Idiopathic Nontoxic Colloid Goiter: Enlarged thyroid glands can occur without iodine deficiency, potentially linked to mild thyroiditis and increased TSH levels.
Abnormalities in thyroid hormone production can arise from enzyme deficiencies affecting iodide trapping, oxidation, coupling, and deiodination.
Certain foods, like some turnips and cabbages, contain goitrogenic substances that can inhibit thyroid function and result in gland enlargement.

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Description

This quiz explores the anatomy, hormones, and functions of the thyroid gland. It covers the structure, composition, and the impact of thyroid hormones on metabolism. Test your knowledge on how these hormones influence the body's metabolic rates and overall health.