L31. Physiology - Thyroid Physiology

Podcast

Play an AI-generated podcast conversation about this lesson

Questions and Answers

What effect does T3 deficiency have on adults?

Listlessness, sleepiness, and weak nervous system function (correct)

Heightened nervous system activity and restlessness

Increased metabolism and weight loss

Enhanced cognitive function and alertness

What condition is associated with intellectual disability if T3 is absent during the first three months of life?

Hyperthyroidism

Congenital hypothyroidism (correct)

Simple goiter

Graves disease

Which of the following symptoms is NOT associated with hyperthyroidism?

Thick and dry skin (correct)

Irritability

Exophthalmos

Increased body temperature

What is the primary cause of simple goiter?

Iodine deficiency (A) Signup and view all the answers

Which condition results from autoimmune destruction of the thyroid gland?

Hashimoto's disease (D) Signup and view all the answers

What regulates the release of thyroid hormones in the HPT axis?

Anterior pituitary hormone TSH (D) Signup and view all the answers

Which characteristic is unique to the thyroid gland compared to other endocrine glands?

It is the only gland requiring an essential element for hormone synthesis. (C) Signup and view all the answers

How do follicular cells change upon stimulation with TSH?

They elongate to increase hormone production. (C) Signup and view all the answers

What is the active form of thyroid hormone that is primarily secreted?

T3 (D) Signup and view all the answers

What is responsible for the storage of thyroid hormone in the thyroid gland?

Colloid, a proteinaceous substance (A) Signup and view all the answers

What type of cells in the thyroid gland are responsible for synthesizing calcitonin?

Parafollicular cells (B) Signup and view all the answers

What is an effect of the hydrophobic nature of thyroid hormones?

They require binding proteins for transport in the blood. (A) Signup and view all the answers

What occurs to the morphology of follicular cells when not stimulated?

They typically have a cuboidal shape. (D) Signup and view all the answers

What percentage of secreted T4 is converted into reverse T3?

45% (B) Signup and view all the answers

Which protein binds to 45-60% of plasma T3?

Thyroxine binding globulin (A) Signup and view all the answers

How much T4 is secreted per day?

90 µg (B) Signup and view all the answers

What is the half-life of T3 in plasma?

1-2 days (B) Signup and view all the answers

What percentage of total circulating T4 is free in plasma?

0.1% (D) Signup and view all the answers

Which gland is stimulated by TSH to release T3 and T4?

Thyroid gland (B) Signup and view all the answers

What is the source of 80% of circulating T3?

Conversion from T4 (D) Signup and view all the answers

Which pathway activates the release of TSH from thyrotrophs?

DAG-PKC pathway (A) Signup and view all the answers

What is the primary function of protein-bound T3/T4 in circulation?

To provide a pool of circulating thyroid hormone (A) Signup and view all the answers

What hormone is released from neurons in the ARN and ME?

TRH (D) Signup and view all the answers

What pathway is activated when TRH binds to thyrotrophs in the anterior pituitary?

DAG-PKC and PLC/IP3 pathway (C) Signup and view all the answers

What is the primary effect of T3 on metabolic activities?

Increased metabolic activity (D) Signup and view all the answers

What is the major reason T3 is considered more biologically active than T4?

T3 has a higher affinity for the receptor than T4 (C) Signup and view all the answers

Which of the following regulates the transcription of genes affected by thyroid hormones?

Thyroid response elements (C) Signup and view all the answers

What can somatostatin and dopamine influence in relation to thyroid hormones?

Inhibit thyrotrophs (B) Signup and view all the answers

What happens to T4 once it enters the target cell?

It is converted to T3 (D) Signup and view all the answers

Which receptor forms a heterodimer with the thyroid hormone receptor (TR)?

Retinoid X receptor (RXR) (A) Signup and view all the answers

What physiological effect is NOT attributed to thyroid hormones?

Decreased gluconeogenesis (C) Signup and view all the answers

Which effect occurs as a response to TSH binding to follicular cells in the thyroid gland?

Release of T3 and T4 (A) Signup and view all the answers

Which of the following correctly describes the feedback mechanism of T3 and T4?

Negative feedback on thyrotrophs (C) Signup and view all the answers

What is the role of the sodium-iodide symporter (NIS) in thyroid hormone production?

It transports iodide into follicular cells. (A) Signup and view all the answers

Which step in thyroid hormone synthesis does TSH not stimulate?

Conjugation of iodinated tyrosines (A) Signup and view all the answers

Which of the following accurately describes the function of thyroid peroxidase in thyroid hormone synthesis?

It catalyzes iodination and conjugation of tyrosines. (C) Signup and view all the answers

What is the significance of the 250:1 gradient mentioned in the context of iodide uptake?

It indicates the efficiency of the sodium-iodide symporter. (C) Signup and view all the answers

Which of the following substances is produced from the proteolytic cleavage of thyroglobulin?

Mono-iodothyronine (MIT) (A) Signup and view all the answers

How does TSH contribute to the growth of follicular cells?

By exerting a hyperplastic effect. (C) Signup and view all the answers

During the initial steps of thyroid hormone production, which compound is first formed from iodide?

Iodine (A) Signup and view all the answers

What is the result of deiodination of MIT and DIT during thyroid hormone synthesis?

Release of free iodide and amino acids. (A) Signup and view all the answers

Which form of thyroid hormone is predominantly synthesized before conversion takes place?

T4 (C) Signup and view all the answers

What is the primary means of transporting colloid containing iodinated thyroglobulin into follicular cells?

Endocytosis (D) Signup and view all the answers

Study Notes

Thyroid Gland Physiology

Learning Objectives: After studying the lecture and notes, students should understand anatomy/morphology of the thyroid gland, thyroid hormone biosynthetic pathway, hypothalamic-pituitary-thyroid axis (HPT axis) and regulation of thyroid hormone release, molecular and cellular action of thyroid hormone, physiological effects of thyroid hormone, and pathologies associated with thyroid dysfunction.

Overview of the Thyroid Hormone System

The thyroid hormone production is regulated by the anterior pituitary hormone TSH, which is itself regulated by the hypothalamic hormone TRH. These three components form the HPT axis.
Increased metabolism, growth and development, and increased catecholamine effect are regulated by thyroid hormones (T3 and T4).

Anatomy and Histology of the Thyroid Gland

The thyroid gland is a palpable endocrine gland with two lobes (left and right) and an isthmus.
Follicular cells synthesize thyroid hormones.
Parafollicular (C) cells produce calcitonin.
Thyroid hormone is stored in colloid.
Iodine is an essential element needed for thyroid hormone production.

Histological Changes in Response to Stimulation by TRH

Follicular cells under basal conditions are low cuboidal.
Stimulation by TSH (thyroid-stimulating hormone) causes follicular cells to become elongated and tall columnar.
Taller cells produce more thyroid hormone.

Structure of Thyroid Hormones

The active form of thyroid hormone is T3.
T4 (thyroxine) is mostly secreted and then converted to T3 in the bloodstream and target cells.
Reverse T3 (rT3) is an inactive form and is only a very small percentage.

Initial Steps in Thyroid Hormone Production

Iodine is transported into the follicular cells via a sodium/iodide symporter (NIS).
Pendrin iodide transporter moves iodine into the lumen.
Thyroid peroxidase (TPO) converts iodide into iodine and bonds it to tyrosine residues of thyroglobulin.

Initial Steps in Thyroid Hormone Synthesis in the Lumen

Iodination by thyroid peroxidase.
Conjugation of iodinated tyrosines by thyroid peroxidase.

Overview of Thyroid Hormone Production and Release

TSH increases iodine trapping.
TSH also stimulate iodination of thyroglobulin in the follicular lumen.
TSH stimulates the conjugation of iodinated tyrosine residues to form T4 and T3, which are then linked to thyroglobulin.
TSH stimulates the endocytosis of iodinated thyroglobulin.
TSH stimulates proteolysis of the iodinated thyroglobulin.
TSH stimulates the secretion of T4 and T3 into the circulation.

Fate of Secreted T4

90 µg of T4 is secreted daily.
T4 is converted to T3.
Some T4 is converted to reverse T3.
T4 serves as a circulating reserve for T3.

Most Thyroid Hormone in Circulation is Bound to Carrier Proteins

Most T3 and T4 are bound to plasma proteins like TBG (thyroxine-binding globulin).
T4 and T3 are bound to albumin and transthyretin.
Only a small percentage (about 0.1% of T4) is unbound/free.

Hypothalamic-Pituitary-Thyroid Axis

Neurons in the hypothalamus and median eminence secrete TRH.
TRH is released into portal veins and binds to receptors on thyrotrophs in the anterior pituitary. This process activates the DAG-PKC and PLC/IP3 pathway, leading to increased protein phosphorylation and release of calcium stores.
Calcium and phosphorylation triggers synthesis and release of TSH from thyrotrophs.
TSH stimulates follicular cells to synthesize and release T3 and T4.
T3 and T4 provide negative feedback on thyrotrophs and TRH-secreting neurons.
Somatostatin and dopamine can inhibit thyrotroph activity, creating a new setpoint for thyroid hormone release.

Cellular Action of Thyroid Hormone

Free T3 and T4 enter target cells.
T4 is typically deiodinated to T3 within the cell.
The thyroid hormone receptor (TR) is a nuclear receptor, similar to steroid hormone receptors.
The receptor binds to DNA at thyroid response elements (TREs).
The binding of T3 and T4 to their receptors regulates the transcription of genes.
T3 is more biologically active than T4 because it is more free-form and has a higher affinity for the receptor.

Physiological Effects of Thyroid Hormone

Thyroid hormone affects basal metabolic rate, carbohydrate metabolism, protein metabolism, lipid metabolism, thermogenesis, and the autonomic nervous system.
Low thyroid hormone levels (hypothyroidism) lead to reduced metabolic activity, decreased protein metabolism, and decreased carbohydrate metabolism.
High levels of thyroid hormone (hyperthyroidism) can lead to increased metabolic activity, increased protein metabolism, and increased carbohydrate metabolism.

Pathologies Associated with the Thyroid Gland

Simple Goiter: Enlarged thyroid gland, typically caused by iodine deficiency.
Hyperthyroidism: Overactive thyroid gland. Symptoms include increased metabolism, weight loss, irritability, and heat intolerance. Common causes include Graves' disease and tumors.
Hypothyroidism: Underactive thyroid gland. Symptoms include decreased metabolism, weight gain, fatigue, and cold intolerance. Common causes include Hashimoto's thyroiditis. Developmental problems can result if hypothyroidism occurs in infants and children.

Studying That Suits You

Use AI to generate personalized quizzes and flashcards to suit your learning preferences.

Description

This quiz covers the anatomy and physiology of the thyroid gland, the biosynthetic pathway of thyroid hormones, and the regulatory mechanisms involving the hypothalamic-pituitary-thyroid axis. Students will explore the physiological effects of thyroid hormones and associated pathologies related to thyroid dysfunction.