Thyroid Hormones: Functions, Mechanism, and Regulation

Thyroid Hormones

Overview

• Thyroid hormones are produced by the thyroid gland, a butterfly-shaped gland located in the neck
• They play a crucial role in regulating metabolism, growth, and development

Types of Thyroid Hormones

• Triiodothyronine (T3): the most active form of thyroid hormone, responsible for regulating metabolism and energy production
• Thyroxine (T4): the less active form of thyroid hormone, converted to T3 in the liver and other tissues
• Reverse T3 (rT3): an inactive form of thyroid hormone, produced in small amounts

Functions of Thyroid Hormones

• Regulate metabolic rate and energy production
• Influence growth and development, particularly in children and adolescents
• Regulate body temperature
• Affect heart rate and cardiac output
• Involved in the regulation of muscle strength and tone

Mechanism of Action

• Thyroid hormones bind to specific receptors in the nucleus of target cells
• This binding activates gene transcription, leading to changes in protein synthesis and cellular metabolism
• Thyroid hormones also have non-genomic effects, influencing cellular signaling pathways and ion channels

Regulation of Thyroid Hormones

• Thyroid-stimulating hormone (TSH) from the pituitary gland stimulates the thyroid gland to produce thyroid hormones
• Thyroid hormones, in turn, feedback to inhibit TSH production
• The hypothalamic-pituitary-thyroid (HPT) axis regulates thyroid hormone production and balance

Clinical Significance

• Hypothyroidism: a condition characterized by low thyroid hormone levels, resulting in fatigue, weight gain, and dry skin
• Hyperthyroidism: a condition characterized by high thyroid hormone levels, resulting in weight loss, anxiety, and rapid heart rate
• Thyroid hormone replacement therapy is used to treat hypothyroidism and thyroid cancer