Thyroid Structure and Hormone Regulation

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Questions and Answers

What is the primary function of the thyroid gland?

Filtering blood

Regulating thyroid hormone production (correct)

Regulating blood pressure

Producing insulin

Match the following components of the thyroid gland with their functions:

Follicular cells = Synthesize and secrete thyroglobulin Parafollicular cells = Secrete calcitonin Thyroid hormones = Increase metabolic rate Iodine transport = Active transport into thyroid follicles

A substance called is contained within the follicles. It is the main storage for hormones and thyroid hormone ____.

A substance called colloid is contained within the follicles. It is the main storage for thyroid hormones and thyroid hormone substrates.

Colloid is rich in thyroglobulin (a glycoprotein)

True Signup and view all the answers

What are the cells that are outside the follicles, which are responsible for secreting calcitonin?

Parafollicular cells / C cells Signup and view all the answers

What are the two precursors involved in thyroid synthesis?

Tyrosine and iodide Signup and view all the answers

Tyrosine is a part of a large protein called thyroglobulin.

True Signup and view all the answers

More specifically, thyroglobulin is produced in the RER of _____ in the thyroid follicles.

More specifically, thyroglobulin is produced in the RER of thyrocytes in the thyroid follicle Signup and view all the answers

What form of iodine is primarily absorbed in the diet?

Iodide (I-) Signup and view all the answers

Which of the following mechanisms drives the active uptake of iodide by follicular cells?

Na+/I- symporters Signup and view all the answers

What role does Na+-K+-ATPase play in iodide uptake?

It establishes a sodium gradient required for iodide uptake. Signup and view all the answers

Which of the following dietary forms of iodine is least likely to be absorbed directly?

Elemental diatomic iodine (I2) Signup and view all the answers

Which of the following statements about iodide is true?

Iodide is absorbed from the diet in its ionic form. Signup and view all the answers

What enzyme in the thyroid follicular lumen will facilitate the synthesis of thyroid hormones?

Thyroid peroxidase Signup and view all the answers

What are the three main functions of thyroid peroxidase?

Oxidation, organification and coupling reaction Signup and view all the answers

Describe what occurs in oxidization (thyroid synthesis)

Thyroid peroxidase oxidizes Iodide (I-) to Iodine (I2) Signup and view all the answers

What occurs during organification (of thyroid hormone synthesis)

As TPO oxidizes iodide to iodine, it concurrently links tyrosine residues to thyroglobulin. This generates the formation of MIT and DIT Signup and view all the answers

Describe what occurs in coupling (aka fusion) of the iodinated tyrosines

During the coupling reaction, TPO combines the MIT/DIT residues to make T3 or T4 Signup and view all the answers

1 MIT + 1 DIT =

T3 or Triiodothyronine Signup and view all the answers

1 DIT + 1 DIT =

T4 or Thyronine or thyroxine Signup and view all the answers

How is the thyroid gland regulated?

It’s regulated via the HPA axis. TRH to TSH to production of T3 an T4 Signup and view all the answers

What influences the regulation of thyroid hormones?

Stress, diet, temperature and time Signup and view all the answers

Large amounts of triiodothyronine (T3) are produced

False Signup and view all the answers

A majority of thyroid hormones are transported in plasma by transporter proteins. What are these three transporter proteins?

Thyroxin binding globulin (in no feline species), transthyretin (in felines) and albumin Signup and view all the answers

What is primarily overproduced in feline hyperthyroidism?

Thyroxine (T4) Signup and view all the answers

What is the most common underlying cause of feline hyperthyroidism?

Functional adenoma Signup and view all the answers

How would you classify feline hyperthyroidism?

Benign condition Signup and view all the answers

Which of the following best describes thyrotoxicosis in the context of feline hyperthyroidism?

Excessive levels of thyroid hormones Signup and view all the answers

Why is T4 preferred over T3 for initial screening of feline hyperthyroidism?

T4 is converted to T3 peripherally by tissue-specific enzymes. Signup and view all the answers

What role do deiodinases play in the metabolism of thyroid hormones in cats?

They convert T4 to T3 in specific tissues. Signup and view all the answers

What is a significant reason for the lack of diiodinase in the thyroid of cats?

It allows for a greater reliance on peripheral T3 production. Signup and view all the answers

What distinguishes the thyroid hormone production in cats from that in other animals?

All of a cat's T3 is derived from peripheral conversion of T4. Signup and view all the answers

Which hormone's level is primarily affected in feline hyperthyroidism?

Thyroxine (T4) Signup and view all the answers

List the general homeostatic actions of thyroid hormones

Thyroid hormones influence growth, basal metabolic rate, metabolism, CNS and cardiovascular system Signup and view all the answers

What is the primary effect of increased thyroid hormones on cellular metabolism?

Increased rate of ATP production Signup and view all the answers

Which thyroid receptors does T3 bind to in target cells to regulate gene expression?

TR𝛼 and TR𝛽 Signup and view all the answers

Which statement about the primary factors for control of metabolism is correct?

Basal metabolic rate (BMR) is a primary factor influenced by thyroid hormones. Signup and view all the answers

In which physiological context does T3's influence lead to protein production?

By enhancing transcription and translation of new proteins Signup and view all the answers

Due to the increase in BMR rate of cells, what metabolisms will it alter?

An increase in BMR will alter carbohydrate, lipid and protein metabolism Signup and view all the answers

What is the molecular action of thyroid hormones on the GI tract?

It increases appetite and food intake (polyphagia), and decrease in fat absorption Signup and view all the answers

What is the effect of thyroid hormones on renal blood flow?

Increased renal blood flow Signup and view all the answers

What happens to the number of aquaporins in the collecting duct when thyroid hormones are present?

Decreased aquaporins, despite high levels of ADH Signup and view all the answers

How do thyroid hormones influence solute excretion in the kidneys?

Increased solute excretion due to enhanced renal perfusion Signup and view all the answers

Which of the following mechanisms is primarily affected by thyroid hormones in the kidneys?

Regulation of aquaporin expression in the nephron Signup and view all the answers

What effect do thyroid hormones have on the overall glomerular filtration rate (GFR)?

They increase GFR through renal blood flow enhancement Signup and view all the answers

What are the two physiological concepts we must consider when we are measuring thyroid hormones?

Thyroid hormones will fluctuate overtime and production of thyroid hormones are affected by other disease processes; therefore a normal thyroid result does not rule out hyperthyroidism Signup and view all the answers

What are the common clinical signs of hyperthyroidism?

Weight loss, polyuria and polydypsia, polyphagia, vomiting, diarrhea, shedding, hyperactivity, tachycardia Signup and view all the answers

What is the etiology behind feline hyperthyroidism?

Primary thyroid gland dysfunction is the most common reason. Secondary hyperthyroidism (pituitary gland dysfunction) is rare Signup and view all the answers

What diagnostics can you use to diagnose a feline with hyperthyroidism?

Screen for T4 and free T4 Signup and view all the answers

What are the four treatment options for feline hyperthyroidism?

Radioactive iodine, oral or transdermal medication, surgical thryoidectomy and dietary therapy Signup and view all the answers

How does methimazole affect thyroid physiology?

It inhibits the function of thyroid peroxidase Signup and view all the answers

What is a direct consequence of decreased thyroid hormone levels in canines?

Decreased metabolic rate of hair follicles Signup and view all the answers

Which of the following best describes how decreased thyroid hormone levels affect cholesterol?

Decreased cholesterol levels in serum Signup and view all the answers

How does hypothyroidism affect hepatic function in canines?

It decreases hepatic uptake of cholesterol Signup and view all the answers

Which physiological change occurs in hair follicles as a result of decreased thyroid hormones?

Altered cycle of hair growth Signup and view all the answers

What role do thyroid hormones primarily play in cholesterol metabolism?

They promote hepatic conversion of cholesterol into bile acids Signup and view all the answers

What is the treatment for dogs with hypothyroidism?

Exogenous thyroxine (T4) Signup and view all the answers

Study Notes

Thyroid Tissue Structure

Composed of numerous sac-like structures known as thyroid follicles.
Thyroid follicles vary in size and play a crucial role in hormone production.
Follicular cells are organized in a circular arrangement within the follicles.

Regulation of Hormones

The thyroid gland controls the production of thyroid hormones through the secretion of Thyroid Stimulating Hormone (TSH).
TSH plays a significant role in stimulating the activity of follicular cells.

Thyroid Hormones

Thyroid hormones, specifically T3 (triiodothyronine) and T4 (thyroxine), are produced from the amino acid tyrosine.
Both T3 and T4 are hydrophobic, allowing them to easily pass through cell membranes.
Thyroid hormones exert their effects by binding to nuclear receptors, influencing gene expression and metabolic activity.

Thyroid Follicles and Hormone Storage

Thyroid follicles are filled with colloid, serving as the primary storage site for thyroid hormones.
Colloid contains thyroglobulin, a protein component that includes the amino acid tyrosine.

Thyroid Hormone Synthesis

Thyroglobulin is synthesized within follicular cells and secreted into colloid for hormone production.
Iodine is absorbed from the diet by follicular cells and actively transported into thyroid follicle cells.
Iodide is trapped within these follicular cells through active transport mechanisms.
The conversion of colloid to active thyroxine occurs within follicular cells, leading to increased metabolic rate, thermogenesis, and gluconeogenesis.

Parafollicular Cells (C-cells)

Parafollicular cells are located outside thyroid follicles, are large, pale, and round in appearance.
They secrete calcitonin in response to elevated plasma calcium levels.
Calcitonin functions to reduce blood calcium levels by decreasing osteoclast activity and increasing renal calcium excretion.

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Description

Explore the intricate structure of thyroid tissue, including the essential thyroid follicles and their role in hormone production. Learn about the regulation of thyroid hormones, including T3 and T4, and their impact on metabolic activity.