Thyroid Hormone Synthesis and Functions

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Questions and Answers

What percentage of thyroid hormone is found free in the circulation?

Around 50%

Less than 1% (correct)

More than 70%

About 10%

What is the primary function of the thyroid gland?

To secrete thyroid hormone and regulate metabolic rate (correct)

To regulate blood pressure

To produce insulin and regulate blood sugar levels

To produce growth hormone and regulate bone growth

What is the primary form of thyroid hormone secreted from the gland each day?

rT3

T4 (correct)

What is the name of the protein found in thyroid colloid?

Thyroglobulin Signup and view all the answers

Which of the following statements about thyroid follicles is true?

They are spherical structures lined with follicular cells Signup and view all the answers

What is the purpose of de-iodination in the metabolism of thyroid hormone?

To convert T4 to T3 Signup and view all the answers

What is the name of the hormone secreted by parafollicular cells?

Calcitonin Signup and view all the answers

What is the name of the specific receptor that T3 binds to in the nucleus?

TR Signup and view all the answers

What is the result of T3 binding to the thyroid hormone receptor in the nucleus?

Increased expression of genes Signup and view all the answers

What is the shape of the thyroid gland?

Butterfly-shaped Signup and view all the answers

What is the main reason thyroid hormones are associated with plasma proteins in the circulation?

Because they are hydrophobic Signup and view all the answers

How many bound iodine atoms are present in triiodothyronine (T3)?

3 Signup and view all the answers

What type of cells synthesize calcitonin?

Parafollicular cells Signup and view all the answers

What is the effect of calcitonin on blood calcium levels?

Decreases blood calcium levels Signup and view all the answers

Which hormone stimulates the secretion of thyroid stimulating hormone (TSH)?

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

What is the function of thyroperoxidase (TPO)?

To oxidize iodide to active iodine Signup and view all the answers

What is the precursor molecule to T3 and T4?

Thyroglobulin Signup and view all the answers

How is T4 formed?

From 2 molecules of DIT Signup and view all the answers

What is the mechanism by which T3 and T4 are released into the bloodstream?

Fusion with lysosomes Signup and view all the answers

What provides negative feedback on the pituitary and hypothalamus?

T3 and T4 Signup and view all the answers

Study Notes

Anatomy of Thyroid Gland

The thyroid gland is one of the largest endocrine glands in the body, with a butterfly-shaped structure consisting of two lobes connected by an isthmus.
It is situated on the front sides of the trachea and located below the larynx.

Thyroid Follicle

The thyroid follicle is the functional unit of the thyroid gland, with a spherical structure lined with follicular cells.
The follicular cavity is filled with a gel-like viscous iodine-rich material called thyroid colloid, which contains a protein called thyroglobulin.

Parafollicular Cells (C Cells)

Parafollicular cells are located between the follicles and secrete calcitonin, which decreases blood calcium levels.

Functions of Thyroid Gland

The thyroid gland secretes two main hormones: thyroxine (T4) and triiodothyronine (T3), which regulate the body's metabolic rate and are necessary for normal growth and development.
The gland also secretes calcitonin, which decreases blood calcium levels.

Synthesis of Thyroid Hormone

Thyroglobulin is synthesized in follicular cells and discharged into the lumen of the follicle.
Iodide is actively taken up from the blood by follicular cells through a sodium-iodide symporter.
Iodide is then oxidized to active iodine (I2) by thyroperoxidase (TPO) and transported into the lumen of the follicle.
Iodine is attached to tyrosine, forming mono-iodotyrosine (MIT) and di-iodotyrosine (DIT), which then link together to form T3 and T4.
T4 is formed from 2 molecules of DIT, while T3 is formed from 1 molecule of DIT and MIT.

Control of Thyroid Hormone Synthesis and Secretion

The thyroid gland is controlled by hormone secretions from the hypothalamic-pituitary-thyroid axis.
The hypothalamus secretes thyrotropin-releasing hormone (TRH), which stimulates the secretion of thyroid-stimulating hormone (TSH) from the anterior pituitary.
TSH stimulates the production of T3 and T4 from the thyroid gland, which in turn exerts negative feedback on the pituitary and hypothalamus.

Transport of Thyroid Hormone

Thyroid hormones are hydrophobic and are found in the circulation associated with plasma-binding proteins, including thyroxine-binding globulin (TBG), pre-albumin, and albumin.
Less than 1% of thyroid hormone is found free in the circulation.

Metabolism of Thyroid Hormone

Approximately 100 μg of thyroid hormones are secreted from the gland each day, mostly in the form of T4 with about 10% as T3.
About 80% of T4 undergoes peripheral conversion to T3 in the liver and kidney, or to reverse T3 (rT3).

Mechanism of Action

T3 enters the nucleus and binds to a specific nuclear thyroid hormone receptor (TR), which binds to DNA at specific sequences called thyroid hormone response elements (TRE).
This increases the expression of various genes that code for proteins that regulate cell functions.

Functions of Thyroid Hormone

Thyroid hormone plays a crucial role in regulating the body's metabolic rate and is necessary for normal growth and development.

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Description

Learn about the synthesis of thyroid hormone and its functions, including the anatomy of the thyroid gland. Part of Medical Biochemistry 2 course.