Thyroid Hormone Synthesis and Functions

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.

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