Thyroid Hormone Synthesis and Release

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Questions and Answers

What is a primary physiological effect of T3 and T4 on skeletal muscle?

Inhibition of glycogen synthesis

Decreased insulin sensitivity

Increased insulin-dependent glucose utilization (correct)

Reduced energy expenditure

How do T3 and T4 influence gene transcription?

By increasing the activity of DNA-dependent RNA polymerase (correct)

By decreasing the activity of RNA polymerase

By actively preventing mRNA synthesis

By binding to cytoplasmic receptors

What is the primary thyroid hormone secreted by the body?

Diiodotyrosine (T2)

Monoiodotyrosine (T1)

Tetraidothyronine (Thyroxine, T4) (correct)

Triiodothyronine (T3)

What role does sodium-potassium ATPase play in the actions of T3 and T4?

It enhances metabolic rate by increasing ATP use. Signup and view all the answers

Which hormone stimulates the release of thyroid-stimulating hormone (TSH)?

Thyrotropin-releasing hormone (TRH) Signup and view all the answers

What is the effect of triiodothyronine (T3) in relation to thyroxine (T4)?

T3 is four times more potent than T4. Signup and view all the answers

What is the role of parathyroid hormone (PTH) in calcium homeostasis?

It regulates plasma Ca2+ by affecting bone, kidneys, and intestines. Signup and view all the answers

How does T3 and T4 exert negative feedback on the hypothalamus and anterior pituitary?

By decreasing TSH secretion. Signup and view all the answers

Which of the following is a result of increased metabolic activity due to T3 and T4?

Increased energy expenditure at rest Signup and view all the answers

What enzyme is particularly involved in the synthesis of T3 and T4?

Thyroid peroxidase Signup and view all the answers

What process does T3 and T4 enhance to increase cellular energy reserves?

Glycogen synthesis Signup and view all the answers

How do T3 and T4 affect vitamin metabolism?

They increase the conversion of beta carotene to vitamin A. Signup and view all the answers

Which process allows iodinated thyroglobulin to enter follicular cells?

Pinocytosis Signup and view all the answers

What physiological role do T3 and T4 hormones play in the body?

Increasing protein synthesis and energy requirements Signup and view all the answers

What effect does T3 and T4 have on adrenal and glucagon lipolytic actions?

They enhance these actions. Signup and view all the answers

What is the relationship between T4 levels and conversion to T3?

Some circulating T4 is converted to T3 by the liver. Signup and view all the answers

What mechanism is primarily responsible for the transport of iodide ions into follicular cells?

Active transport Signup and view all the answers

Which of the following steps is NOT involved in the synthesis of T3 and T4?

Conversion of T3 to T4 Signup and view all the answers

What role does thyroid peroxidase play in the synthesis of thyroid hormones?

It catalyzes the iodination of tyrosine Signup and view all the answers

Which hormone is primarily responsible for regulating blood calcium levels?

Calcitonin Signup and view all the answers

Which statement correctly describes the physiological effects of T3 and T4?

They regulate metabolism and promote growth and development Signup and view all the answers

What is the function of the colloid present in the thyroid gland follicles?

Storage of thyroglobulin Signup and view all the answers

What is a likely consequence of decreased secretion of T3 and T4 from the thyroid gland?

Decreased metabolic activity Signup and view all the answers

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

Synthesis from thyroglobulin and iodide Signup and view all the answers

Study Notes

Thyroid Hormone Synthesis, Storage and Release

Thyroid hormones are produced by the thyroid gland and are essential for regulating metabolism, growth, and development.
Thyroid hormones are synthesized from tyrosine, an amino acid found in thyroglobulin.
The thyroid gland has follicles, lined with follicular cells, which produce thyroglobulin and secrete it into the colloid.
Colloid is a viscous fluid, stored in the follicles, and contains thyroglobulin.
Thyroid hormones T3 (triiodothyronine) and T4 (thyroxine) are synthesized and stored in the colloid as part of the thyroglobulin molecule.
T4 is the main hormone secreted (~90%).
T3 is about 4 times more potent than T4 on target tissues.
T3 and T4 are predominantly transported in the circulation bound to carrier molecules.

Steps involved in the synthesis and release of thyroid hormones:

Step 1. Uptake of Iodide: Iodide ions are actively transported from the circulation into the follicular cells, creating a higher concentration inside the cells compared to surrounding fluid.
Step 2. Oxidation of Iodide: Iodide ions are oxidized inside the follicular cells by thyroid peroxidase to generate either iodinium ions (I+) or iodide free radicals.
Step 3. Iodination of Tyrosine: The iodinium ions are attached to the tyrosine ring of tyrosines within the thyroglobulin molecule via thyroid peroxidase.
Step 4. Coupling of Iodotyrosines: Monoiodotyrosine (T1) and Diiodotyrosine (T2) are coupled via covalent bonds on thyroglobulin to form T4 and T3.
Step 5. Proteolytic Cleavage of Thyroglobulin: The iodinated thyroglobulin is taken into the follicular cells by pinocytosis, where it fuses with a lysosome.
Step 6. Release of T3 and T4: Lysosomal enzymes break down thyroglobulin, releasing T3 and T4 and any remaining T1 and T2. T1 and T2 are recycled, and T3 and T4 are released into the circulation.

Regulation of T3 and T4 Secretion

Thyroid-stimulating hormone (TSH) from the anterior pituitary gland regulates the synthesis and release of T3 and T4.
Increased demand for T3 and T4 triggers signals in the hypothalamus, leading to the release of thyrotropin-releasing hormone (TRH).
TRH stimulates the anterior pituitary to increase TSH secretion.
TSH binds to receptors on the follicular cells of the thyroid gland, increasing iodide uptake, thyroglobulin synthesis and thyroid peroxidase levels.
Increased circulating levels of T3 and T4 provide negative feedback to the hypothalamus and anterior pituitary, decreasing TRH and TSH release.
Some circulating T4 is converted to T3 by the liver.
In children, cold temperatures stimulate TRH secretion, helping them adapt to changes in environmental temperatures.

Actions of T3 and T4 on Metabolism

T3 and T4 increase protein synthesis, building and repairing muscle, which requires more energy and oxygen.
T3 and T4 increase basal metabolic rate: increased oxygen consumption and energy expenditure at rest. This also generates more heat.
T3 and T4 alter metabolism:
- Increase vitamin demand
- Enhance the lipolytic action of adrenaline and glucagon, releasing lipids for energy.
- Increase insulin-dependent glucose utilization in target tissues (like skeletal muscle) to meet increased energy demand and stimulate glycogen synthesis.
T3 and T4 are lipophilic and cross cell membranes freely.
T3 and T4 bind to receptors in the nucleus of cells, increasing DNA-dependent RNA polymerase. This increases gene transcription and mRNA production, leading to increased protein synthesis of enzymes such as respiratory enzymes.
T3 and T4 also increase the production of sodium-potassium ATPase. This pump expels sodium ions from cells in exchange for potassium ions, increasing ATP use and overall metabolism.

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Description

Explore the intricate process of thyroid hormone synthesis, storage, and release. This quiz covers the essential functions of thyroid hormones, their production in the thyroid gland, and the steps involved in their synthesis. Test your knowledge on T3 and T4, and the role of iodide in hormone production.