58. Disturbances in the function of the thyroid gland.

DISTURBANCES IN THE FUNCTIONS OF THE THYROID GLAND

• The thyroid gland consists of 2 types of cells:

1. Follicular cells - produce hormones T3 and T4

• T3 (triiodothyronine) - active form
• T4 (thyroxine) - prohormone → converted to T3 in peripheral tissues with cofactor selenium

2. Parafollicular cells - secrete thyrocalcitonin

• Thyrocalcitonin decreases Ca²⁺ levels by inhibiting osteoclasts and decreasing the absorption of Ca²⁺ in the intestinal tract

Control of secretion

• Secretion is regulated by the hypothalamic-pituitary-thyroid feedback system
• Thyrotropin-releasing hormone (TRH) which is produced by the hypothalamus, controls the release of thyroid-stimulating hormone (TSH) from the anterior pituitary gland
• TSH then stimulates the activity of the thyroid gland by promoting the release of thyroid hormone into the bloodstream and increasing T3 and T4 synthesis
• Increased levels of T3 and T4 then eventually inhibits the secretion of TRH via negative feedback

Actions of thyroid hormones

• Metabolic rate
• Thyroid hormone increases the basal metabolic rate
• Increases metabolism of proteins, fats, and carbohydrates
• Cardiovascular function
• Vasodilation to dissipate body heat due to higher metabolism
• Blood volume, cardiac output, and ventilation are increased
• Heart rate and cardiac contractility are enhanced
• Gastrointestinal function
• Increase in gastric motility and production of GI secretions → diarrhea
• Increase in appetite and food intake
• Influences body weight
• Neuromuscular effects
• Effects neural control of muscle function and tone
• Essential for normal brain development and growth