Thyroid Hormone Synthesis and Secretion

Overview

• Understanding the synthesis and secretion of thyroid hormone
• Regulation of this process through feedback mechanisms

Hypothalamus and Hormone Secretion

• Neurosecretory neurons synthesize and secrete hormones
• Hormones travel to the anterior pituitary

Thyrotropin-Releasing Hormone (TRH)

• Synthesized and secreted by neurosecretory neurons
• Travels down the axon to the axon terminal
• Released into the hypophyseal portal system
• Bridges the hypothalamus and anterior pituitary

Anterior Pituitary and Thyroid-Stimulating Hormone (TSH)

• TRH binds to receptors on endocrine cells in the anterior pituitary
• Stimulates synthesis and secretion of TSH
• TSH enters systemic circulation

Action of TSH

• Targets cells of the thyroid gland
• Promotes synthesis and secretion of thyroid hormones (TH)

Thyroid Hormones: T3 and T4

• T3 (Triiodothyronine) contains three iodides
• T4 (Thyroxine) contains four iodides
• T4 is more prevalent, T3 is more potent
• T4 converts to T3 in systemic circulation

Functions of Thyroid Hormones

• Increase mitochondria in cells
• Control rate of cellular respiration and metabolism
• Increase sodium-potassium ATPase activity
• Increase basal metabolic rate and body heat production

Target Tissues

• Liver & Skeletal Muscle: Stimulate glycogenolysis, increasing plasma glucose
• Adipose Tissue: Stimulate lipolysis, increasing fatty acid and glucose levels
• Skeletal Muscle: Cause proteolysis, increasing amino acid plasma levels

Feedback Mechanisms

• Negative feedback regulates TH synthesis and secretion
• TH inhibits anterior pituitary's response to TRH
• TH inhibits synthesis and secretion of TRH in the hypothalamus

Summary

• Thyroid hormone contributes to growth and development
• Maintains balance between catabolic and anabolic reactions
• Uses feedback mechanisms to regulate hormone levels