Thyroid Gland Anatomy and Physiology PDF

**[THYROID GLAND]** - Lies inferior to thyroid cartilage of larynx. - Consists of two **lobes** connected by narrow **isthmus**. - **Thyroid** **follicles** - Hollow spheres lined by cuboidal epithelium. - Surrounded by capillaries. - Cells absorb **iodi...

**[THYROID GLAND]** - Lies inferior to thyroid cartilage of larynx. - Consists of two **lobes** connected by narrow **isthmus**. - **Thyroid** **follicles** - Hollow spheres lined by cuboidal epithelium. - Surrounded by capillaries. - Cells absorb **iodide ions** (**I^--^**) from blood. - **Follicle** **cavity** contains viscous colloid. - **C** (**clear**) **cells**, or parafollicular cells **Thyroglobulin** - Globular protein synthesized by follicle cells. - Secreted into colloid of thyroid follicles. - Contains the amino acid, **[tyrosine]** - The building block of thyroid hormones. **Thyroid hormones** - **[Thyroxine (T4)]**, or tetraiodothyronine - Contains four iodine atoms. - **[Triiodothyronine (T3)]** - Contains three iodine atoms. **Thyroid-binding globulins (TBGs)** - Proteins that bind about 75 percent of T4 and 70 percent of T3 entering the bloodstream. - Transthyretin and albumin - Bind most of the remaining thyroid hormones. - About 0.3 percent of T3 and 0.03 percent of T4 remain unbound and free to diffuse into tissues. **Thyroid-stimulating hormone (TSH)** - Absence causes thyroid follicles to become inactive. - Neither synthesis nor secretion occurs. - Binds to plasma membrane receptors. - Activates key enzymes in thyroid hormone production. Thyroid hormones affect almost every cell in body. - Enter target cells by transport system - Bind to receptors - In cytoplasm - On surfaces of mitochondria - In nucleus - In children, essential to normal development of skeletal, muscular, and nervous systems. Thyroid hormones activate genes involved in glycolysis and ATP production. - Results in **[calorigenic effect]**. - Increased energy consumption and heat generation of cells. - Responsible for strong, immediate, and short-lived increase in rate of cellular metabolism. **[C cells:]** produce calcitonin (CT). - Helps regulate concentrations of Ca2+ in body fluids. - Stimulates Ca2+ excretion by kidneys. - Prevents Ca2+ absorption by digestive tract. **Effects of thyroid hormones** - Elevate oxygen and energy consumption; in children, may cause rise in body temperature. - Increase heart rate and force of contraction. - Increase sensitivity to sympathetic stimulation. - Maintain normal sensitivity of respiratory centers to oxygen and carbon dioxide concentrations. - Stimulate red blood cell formation. - Stimulate activity in other endocrine tissues. - Accelerate turnover of minerals in bone. 18-5 The four parathyroid glands secrete parathyroid hormone, which increases the blood calcium ion level. **[PARATHYROID GLANDS ]** - Two pairs. - Embedded in posterior surface of thyroid gland. - Altogether, the four glands weigh 1.6 g. Parathyroid hormone (PTH), or parathormone - Secreted by parathyroid (principal) cells in response to low concentrations of Ca2+ in blood. - **Antagonist for calcitonin**. Major effects of parathyroid hormone - Stimulates osteoclasts. - Accelerates mineral turnover and Ca2+ release. - Enhances reabsorption of Ca2+ by kidneys, reducing urinary losses. Stimulates formation and secretion of calcitriol by kidneys