Wk1 Thyroid Gland Function Overview

Question 1

What are the five major categories of physiologic functions of the endocrine system?

Answer 1

1. Regulation of energy balance
2. Control of fuel mobilization
3. Utilization
4. Storage
5. Metabolic demands

The hormones produced by the thyroid gland are responsible for functions in the third category.

Question 2

The thyroid and parathyroid glands arise from which pharyngeal pouches during development?

Answer 2

Third and fourth pharyngeal pouches

This development results in small endocrine organs located near the cranial trachea.

Question 3

Where are thyroid glands typically located in mammalian species?

Answer 3

Superficial neck, adjacent to the trachea, just caudal to the thyroid cartilage of the larynx

There is variation in gland morphology and location between species.

Question 4

True or false: The bovine thyroid gland is typically a single bilobed structure.

Answer 4

TRUE

It is connected by a bit of thyroid tissue across the ventral trachea.

Question 5

What are the two distinct cell types found in the thyroid gland?

Answer 5

• Follicular cells
• Medullary cells

Follicular cells line follicles and are involved in hormone production.

Question 6

What do chief cells in the parathyroid glands produce?

Answer 6

Parathyroid hormone

Chief cells are densely packed and smaller than thyroid gland cells.

Question 7

What is the function of medullary cells in the thyroid gland?

Answer 7

Production and secretion of calcitonin

Medullary cells are also known as parafollicular cells.

Question 8

What is the hormonal axis that controls thyroid hormone synthesis?

Answer 8

Hypothalamus, pituitary, and thyroid glands

The hypothalamus secretes thyrotropin releasing hormone (TRH).

Question 9

What does TSH stand for?

Answer 9

Thyroid stimulating hormone

TSH stimulates production and release of thyroid hormones.

Question 10

What is the half-life of thyroid stimulating hormone (TSH)?

Answer 10

Approximately 45 minutes

TSH is particularly relevant for evaluating thyroid disorders.

Question 11

What is thyroglobulin?

Answer 11

A tyrosine-based glycoprotein synthesized in the rough endoplasmic reticulum

It is utilized by thyroid follicular cells for hormone production.

Question 12

What is the process by which iodide is transported into the follicular lumen?

Answer 12

Iodide chlorine transport systems called pendrine

This occurs after iodide is taken in from the blood.

Question 13

What are the inactive thyroid hormone precursors called?

Answer 13

PRE T3 and PRE T4

These precursors are stored in the colloid until activated.

Question 14

What is the primary function of thyroid hormones T3 and T4?

Answer 14

Regulate metabolism

T3 is more potent, while T4 is more prevalent in circulation.

Question 15

What is the half-life of T4?

Answer 15

Approximately 7 days

T4 accounts for about 90% of all thyroid hormones in circulation.

Question 16

What is the half-life of T3?

Answer 16

Approximately 2 to 3 days

T3 binds to thyroid hormone receptors with greater affinity.

Question 17

Where are thyroid hormone receptors located?

Answer 17

In the nucleus

They bind to intranuclear receptors on target genes.

Question 18

What are the five main categories of functions targeted by thyroid hormones?

Answer 18

• Systemic
• Metabolic
• Cardiovascular
• Pulmonary
• Developmental
• Nervous

These functions include increasing basal metabolic rate and enhancing protein synthesis.

Question 19

What effect does thyroid hormone have on cardiac output?

Answer 19

Increases cardiac output and potentially raises blood pressure

This is due to increased heart rate and force of contractions.

Question 20

What is the role of glucocorticoids like cortisol in relation to T4?

Answer 20

Displace protein-bound hormones like T4

This can lead to issues in diseases with excessive glucocorticoid hormones.

Question 21

What are the metabolic effects of thyroid hormone?

Answer 21

• Increases basal metabolic rate
• Enhances protein and fat metabolism
• Increases carbohydrate absorption

These effects lead to increased protein synthesis and cellular uptake of glucose, lipids, and amino acids.

Question 22

Thyroid hormone increases the heart rate and force of contractions in which system?

Answer 22

Cardiovascular system

This results in increased cardiac output and potentially raises blood pressure.

Question 23

Thyroid hormone is essential for normal development of which systems?

Answer 23

• Musculoskeletal system
• Nervous system

It also increases the resting respiratory rate.

Question 24

True or false: Thyroid hormone affects the sympathetic nervous system responsible for the fight or flight response.

Answer 24

TRUE

Understanding thyroid hormone effects helps in understanding hyperthyroidism and hypothyroidism.

Question 25

What is the role of **cortisol** in relation to T4?

Answer 25

* Highly protein-bound hormone * Displaces T4 from plasma proteins ## Footnote Excess glucocorticoids can lead to decreased total T4 while free T4 may remain normal.

Question 26

What is **Euthyroid 6 Syndrome**?

Answer 26

Low total T4 in the absence of hypothyroidism ## Footnote It is commonly caused by glucocorticoid excess.

Question 27

What are the two types of **endocrine disorders** based on function?

Answer 27

* Hyperfunction (excessive production) * Hypofunction (decreased production) ## Footnote These disorders can be classified as primary or secondary.

Question 28

Define a **primary disorder** in the context of endocrine glands.

Answer 28

A problem with the endocrine gland itself leading to hyper or hypofunction ## Footnote Example: A lesion in the thyroid gland causing hyperthyroidism.

Question 29

Define a **secondary disorder** in the context of endocrine glands.

Answer 29

The endocrine gland is normal, but its function is altered due to another gland's problem ## Footnote Example: Excess thyroid stimulating hormone from the pituitary affecting the thyroid gland.

Question 30

What is the **half-life** of T4?

Answer 30

Seven days ## Footnote This means it takes time for T4 levels to decrease noticeably in blood work.

Question 31

What happens to free T4 levels during **chronic excessive cortisol** exposure?

Answer 31

Free T4 levels return to normal while total T4 decreases ## Footnote This occurs without changes to free T4 due to the prolonged effect of cortisol.

Question 32

What is the significance of measuring **free T4**, **bound T4**, and **total T4**?

Answer 32

* Free T4: biologically active * Bound T4: protein-bound * Total T4: sum of bound and free T4 ## Footnote Understanding these measurements is crucial for diagnosing thyroid disorders.

Question 33

What is the term for **excessive production** of thyroid hormones?

Answer 33

Hyperthyroidism ## Footnote This can be caused by a lesion on the thyroid gland.

Question 34

What is the term for **decreased production** of thyroid hormones?

Answer 34

Hypothyroidism ## Footnote This is often misdiagnosed in dogs due to glucocorticoid effects.

Question 35

What does the term **pituitary dependent hyperthyroidism** refer to?

Answer 35

Secondary thyroid hyperfunction due to excessive TSH from the pituitary ## Footnote This is a result of a lesion in the pituitary gland.

Question 36

What is the **physiological effect** of glucocorticoids on thyroid hormone release?

Answer 36

Suppresses the release of thyroid stimulating hormone (TSH) ## Footnote This leads to decreased circulating T3 and T4 levels.