Thyroid Gland Anatomy and Function Quiz

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Questions and Answers

Where is the thyroid gland located?

Below the thyroid cartilage (correct)

Above the hyoid bone

In front of the cricoid cartilage

Beside the carotid arteries

What are the structural components of the thyroid gland?

Two separate lobes with no connection

Two lobes joined by an isthmus (correct)

Four lobes each connected by a duct

A single lobe and a non-palpable segment

What could happen to the pyramidal lobe of the thyroid gland?

It is removed during thyroid surgery

It may enlarge with disease (correct)

It always remains prominent

It is present in all adults

Which cartilage structure is directly located above the thyroid gland?

Thyroid cartilage Signup and view all the answers

What is the significance of the isthmus in the thyroid gland?

It connects the two lobes of the gland Signup and view all the answers

What role does the sodium-iodide symporter (NIS) play in iodide trapping?

It actively transports iodide from the bloodstream into thyroid follicular cells. Signup and view all the answers

Which statement about Pendrin is true?

Pendrin is implicated in Pendred syndrome, a form of congenital hypothyroidism. Signup and view all the answers

What is the effect of TSH on iodide trapping in thyroid cells?

TSH stimulates the activity of the sodium-iodide symporter. Signup and view all the answers

Which of the following substances serves as an inhibitor of iodide uptake?

Perchlorate (ClO4-) Signup and view all the answers

What process converts iodide (I-) to iodine (I2)?

Oxidation/Organification by TPO. Signup and view all the answers

What role does increased gluconeogenesis play in excess conditions?

Degrades proteins Signup and view all the answers

Which physiological system is critical for normal CNS neuronal development?

Endocrine system Signup and view all the answers

How does growth hormone affect skeletal muscle contraction?

Increases rate and force of contraction Signup and view all the answers

What is one of the specific actions of growth hormone regarding bone development?

Promotes calcification of bones Signup and view all the answers

Which of the following is NOT a characteristic effect of growth hormone on the body?

Decreases oxygen consumption Signup and view all the answers

What is the role of the thyroid hormone receptor in the context of gene expression?

It activates transcription by binding to the thyroid response element of DNA. Signup and view all the answers

Which of the following is NOT a metabolic effect of the thyroid hormone?

Decreased oxygen consumption Signup and view all the answers

How does thyroid hormone influence the cardiovascular system?

It enhances heart strength and increases cardiac output. Signup and view all the answers

Which process is NOT stimulated by thyroid hormones in the context of fat metabolism?

Enhanced fat storage in adipose tissue Signup and view all the answers

What is one of the systemic effects of thyroid hormone associated with increased metabolic activity?

Increased basal metabolic rate Signup and view all the answers

What is the primary functional unit of the thyroid gland?

Thyroid follicle Signup and view all the answers

Which type of cells within the thyroid gland are responsible for producing calcitonin?

C-Cells (Parafollicular cells) Signup and view all the answers

From which arteries does the arterial supply for the thyroid gland primarily originate?

Superior and inferior thyroid arteries Signup and view all the answers

What role do capillaries play in the thyroid gland?

Deliver nutrients and transport hormones Signup and view all the answers

At which stage of gestation does the thyroid gland first begin to develop?

3rd week Signup and view all the answers

What structure connects the thyroid gland to the tongue during its development?

Thyroglossal duct Signup and view all the answers

What potential complication may arise from damage to the recurrent laryngeal nerve?

Vocal cord paralysis Signup and view all the answers

Which of the following nerves is primarily responsible for the innervation of the thyroid gland?

Recurrent laryngeal nerve Signup and view all the answers

What is the primary form of thyroid hormone found in circulation?

T4 Signup and view all the answers

Which enzyme is responsible for converting T4 to the inactive form rT3?

Inner-ring monodeiodinase Signup and view all the answers

How does T3 influence TSH secretion from the pituitary gland?

It inhibits TSH release Signup and view all the answers

What is the role of TRH in thyroid hormone regulation?

It stimulates the release of TSH from the pituitary Signup and view all the answers

What happens when the body has sufficient T3 levels?

The inner-ring enzyme is activated to produce rT3 Signup and view all the answers

Which of the following is a direct effect of TSH on the thyroid gland?

Increased activity of thyroid cells Signup and view all the answers

What are the conditions under which TSH levels would increase?

Decreased levels of thyroid hormones Signup and view all the answers

Which factor does NOT affect the synthesis of thyroid hormones?

Level of somatostatin Signup and view all the answers

What defines T4's role as a prohormone?

It is converted to T3, which mediates effects. Signup and view all the answers

What is the consequence of excessive TSH levels?

Hypertrophy of thyroid cells Signup and view all the answers

Study Notes

Thyroid Hormone Synthesis, Transport and Cellular Mechanism

The thyroid gland is located below the thyroid cartilage, behind the strap muscles.
It consists of two lobes connected by an isthmus, situated below the cricoid cartilage.
A pyramidal lobe is present in about 50% of adults; it's not usually palpable and can enlarge with disease.

Learning Outcomes

Describe the structure and location of the thyroid gland.
Describe the origin of the thyroid gland and its functional relationships.
Explain the mechanisms and control of thyroid hormone synthesis.
Outline the biochemical and clinical functions of thyroid hormones.

Thyroid Gland: Location and Structure

The thyroid gland is situated below the thyroid cartilage.
The thyroid gland comprises two lobes connected by an isthmus.
The isthmus of the thyroid gland is located below the cricoid cartilage.
A pyramidal lobe is present in ~50% of adults.
It is not usually palpable.
It can enlarge with disease.

Thyroid Gland & Follicles

The functional unit of the thyroid gland is the thyroid follicle or acinus.
Thyroid follicles consist of follicular epithelial cells and a lumen filled with colloid.
Capillaries deliver nutrients and transport hormones.
The sympathetic nervous system influences hormone synthesis and secretion.
Lymphatic vessels drain excess fluid.
Parafollicular cells (C-cells) produce calcitonin.

Thyroid Gland: Blood Supply & Innervation

Arterial Supply:
- Superior thyroid artery (from external carotid)
- Inferior thyroid artery (from thyrocervical trunk)
Venous Drainage:
- Superior and middle thyroid veins (drain into the internal jugular vein)
- Inferior thyroid vein (drains into the brachiocephalic vein)
Innervation:
- Recurrent laryngeal nerve (branch of vagus)
- Damage to this nerve can cause vocal cord paralysis.
- The cricothyroid membrane can be used for emergency cricothyrotomy.

Thyroid Gland Origin - Embryology

The thyroid is the first endocrine gland to develop (around the 24th day of gestation).
It originates from the first pharyngeal arch.
The thyroid precursor develops from the thyroglossal duct.
This duct is a diverticulum appearing in the floor of the pharynx.
It grows and descends in the neck as an initially hollow structure then solidifies and becomes bilobed, connected by an isthmus.
The thyroglossal duct breaks down by the late fifth week.
The initial descent is anterior to the pharyngeal gut; it remains connected to the tongue by the thyroglossal duct before bifurcating into two lobes.
The thyroid gland is positioned in its final location, anterior to the trachea by the seventh week of gestation.
The thyroglossal duct obliterates completely by the 7th to 10th week.
Remnants of the duct may persist.
Ectopic thyroid tissue can arise along this path.

Ectopic Thyroid Gland

Ectopic thyroid tissue can arise in abnormal locations along the path that the thyroglossal duct descends.
An example includes the lingual thyroid.

Thyroid Hormone: An Overview

Thyroid hormones (T3 and T4) are synthesized from iodine and tyrosine residues of thyroglobulin.
The synthesis of T3 and T4 need the elements iodine (I) and tyrosine.
"Reverse T3" (rT3) is not biologically active.

What is Thyroid Hormone?

Thyroid hormones are critical for brain and skeletal development and growth in infants.
They regulate metabolic activity in most tissues, except for brain, spleen, and testes.
Thyroid hormones increase basal metabolic rate and oxygen consumption.
They increase heat production by stimulating Na+-K+ ATPase.
They affect the functions of virtually all tissues.

Thyroid Hormone Synthesis - Building Blocks

Iodine
Thyroglobulin
Tyrosine

Iodine

Iodine is a micronutrient found in seafood, dairy products, and grains & vegetables.
The recommended daily intake varies based on age and pregnancy status.
Iodized salt has been crucial in the prevention of iodine deficiency disorders.

Endemic Goiter

In areas where the soil is deficient in iodine, hypothyroidism is common.
The thyroid gland enlarges (forms a goiter) to produce more thyroid hormone to compensate for the lack of iodine.
Worldwide, table salt (NaCl) is enriched with iodine (iodized salt) to prevent iodine deficiency.

Cretinism in Endemic Severe Iodine Deficiency

Thyroid hormone is essential for fetal and postnatal brain as well as skeletal maturation.
Iodine deficiency during pregnancy in both the mother and the baby can result in coarse facial features, umbilical hernia, large fontanelles, and macroglossia at birth.
Continued iodine deficiency after birth can lead to severe growth and cognitive/neurological impairments.

Thyroid Hormone Synthesis - Building Blocks (cont'd)

Thyroglobulin: large glycoprotein produced by the rough endoplasmic reticulum of thyroid follicular epithelial cells.
It is stored in vesicles and exocytosed into the colloid.
Composed of two subunits.
Contains tyrosine residues sterically oriented for thyroid hormone production.

Synthesis of Thyroid Hormones

Steps of synthesis include iodide trapping, oxidation/organification, coupling, endocytosis, proteolysis, transport, and action on receptors.

Synthesis of Thyroid Hormones (cont'd): Iodide Trapping

The basal membrane of thyroid follicular endothelial cells actively transport iodide from the bloodstream against the gradient.
A sodium-iodide symporter (Na+/K+ ATPase) is involved in this active transport.
Perchlorate anions are inhibitors of iodide uptake clinically.
Pendrin transports iodide into the colloid.

Synthesis of Thyroid Hormones (cont'd): Oxidation/Organification

Iodide (I-) is converted to iodine (I2) with H2O2.
The enzyme thyroid peroxidase (TPO) catalyzes this process.
Iodide combines with tyrosine residues of thyroglobulin, also catalyzed by TPO.
Iodotyrosine products generated are monoiodotyrosine (MIT) and diiodotyrosine (DIT).

Synthesis of Thyroid Hormones (cont'd): Coupling

MIT and DIT combine to form triiodothyronine (T3) and thyroxine (T4).
Thyroid peroxidase (TPO) catalyzes this action.
Iodinated thyroglobulin is stored in the colloid.
Colloid stores enough thyroglobulin for 2-3 months of thyroid hormone needs.

Secretion of Thyroid Hormones: Endocytosis and Proteolysis

Colloid is pinocytosed into the follicular cell prior to secretion.
Lysosomal proteases digest thyroglobulin, releasing T3 and T4.
MIT and DIT are degraded, and the iodine is re-used.
T4 and T3 diffuse across the basolateral membrane into capillaries and enter the circulation.

Transport of Thyroid Hormones

Thyroid hormones are hydrophobic and need carrier proteins (like thyroxine-binding globulin (TBG), thyroxine-binding prealbumin (TBPA), and serum albumin) to travel in the bloodstream.
T4 binds more strongly to serum binding proteins compared to T3, where only a small percentage circulate freely.

Thyroid Hormone Circulates Bound to Serum Proteins

Almost all thyroid hormone (T3 and T4) is transported bound to carrier proteins.
Only a tiny percentage of T3 and T4 circulates freely in the blood.

Conversion of Iodothyronines

The thyroid gland mainly secretes T4.
T3 is 2-10 times more biologically active than T4.
Peripheral tissues (especially the liver and kidneys) convert T4 into more active T3.
T4 serves as a prohormone to T3 producing it through deiodinase enzymes.
Various percentages of T3 and T4 production occur from peripheral tissues.

Thyroid is Under Hypothalamic/Pituitary Control (Negative Feedback Loop)

Hypothalamus releases TRH.
TRH stimulates the anterior pituitary to release TSH.
TSH stimulates the thyroid gland to release T3 and T4.
T3 and T4 exert negative feedback on both the anterior pituitary and the hypothalamus to regulate the production process (reducing further TRH and TSH release).

Regulation of Thyroid Hormone Synthesis

Factors influencing thyroid hormone synthesis include hypothalamic TRH, somatostatin (SS), anterior pituitary TSH, iodine availability, thyroid gland integrity, and T4 to T3 conversion in tissues.

Thyroid Stimulating Hormone (TSH)

TSH is a glycoprotein produced in the anterior pituitary.
It is secreted in pulses, with slightly higher levels at night.
TSH stimulates thyroid hormone synthesis, leading to increased size and vascularity of the thyroid gland.
Increased levels of TH negatively feedback on TSH release.
Conversely, decreased TH levels increase TSH release.
Iodine deficiency causes goiter.

Thyroid Response to TSH

TSH causes an increase in cAMP, Ca²⁺, and protein kinase activity.
Increased uptake of colloid results from increased TSH activity.
Increased liberation of T3 and T4 occurs.
Increased production of thyroglobulin is stimulated by TSH.
Increased uptake of iodine is stimulated.
Increased iodination of thyroglobulin is stimulated.
TSH causes increased size and activity of thyroid cells and increased thyroid cell numbers (hypertrophy).

Inhibitors (Drugs and Goitrogens) of Thyroid Hormone Synthesis

Various substances inhibit various steps of thyroid hormone synthesis.
Iodine and thyroid peroxidase activity are inhibited.
Blocking any step in thyroid hormone synthesis can result in various changes in the thyroid functions with relevant clinical consequences.

Tests of Thyroid Function

Thyroid-stimulating hormone (TSH) measures pituitary's response to T3 and T4 levels.
Free thyroxine (FT4) measures unbound thyroxine in the blood.
Free triiodothyronine (FT3) measures unbound triiodothyronine in the blood.
Total thyroxine (T4) and total triiodothyronine (T3) measure both bound and free hormones in the blood.

Mechanism of Action of Thyroid Hormone

T4 is converted to T3.
T3 binds to thyroid receptors (TR) along with retinoid X receptors (RXR).
The resulting complex binds to the thyroid response element (TRE) on DNA.
This process regulates transcription of target genes by influencing the expression of various proteins that subsequently affect cellular responses.

Effects - Metabolic

Thyroid hormones stimulate carbohydrate metabolism (increased glucose uptake, enhanced glycolysis, gluconeogenesis, insulin secretion).
They stimulate fat metabolism (increased lipolysis, decreased fat stores, increased plasma free fatty acids, increased β-oxidation).
They affect mitochondrial number, Na+/K+ ATPase activity, oxygen consumption, protein synthesis, and basal metabolic rate (BMR).
Thyroid hormone levels affect cholesterol, phospholipids and TAG levels in the plasma.
Increase or decrease in levels of cholesterol are linked to specific conditions that arise from thyroid dysfunction.

Effects - Systemic

Thyroid hormones increase blood flow and cardiac output, heart rate, and heart strength.
Thyroid hormones affect the cardiovascular system via their effects on O₂ demand.
Increase in respiration and enhance wakefulness, alertness, memory, learning, and emotional tone.
They affect speed and amplitude of peripheral nerve reflexes.

Effects - Growth & Development

Thyroid hormones stimulate growth and maturation of bone, teeth, epidermis, hair follicles, and nails.
They increase skeletal muscle contraction rate and force.
They are essential for fetal central nervous system development.

Summary of Actions

Thyroid hormones influence central nervous system development and growth including body growth.
They regulate basal energy economy by influencing metabolic actions.
They affect intermediary metabolism through carbohydrate, lipid, and protein metabolism.
Thyroid hormones influence TSH secretion through feedback interactions.

Case Presentations

Case study examples of primary hyperthyroidism and primary hypothyroidism are presented.
Test results and symptoms for both conditions are included as examples of case presentations.

MCQ

Questions related to thyroid hormone function are presented and their correct answers are included.

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Description

Test your knowledge on the anatomy and physiology of the thyroid gland. This quiz covers the location, structural components, and roles of various proteins and hormones associated with thyroid function. Perfect for students studying endocrinology or anatomical sciences.