Overview
This lecture covers the anatomy, histology, and functions of the thyroid and parathyroid glands, their hormones, and the regulation of calcium and metabolism in the body.
Thyroid Gland Structure and Function
- The thyroid gland is highly vascularized and located inferior to the larynx, wrapping around the trachea in a C-shape.
- Composed of spherical thyroid follicles lined by follicular (cuboidal epithelial) cells and filled with colloid (thyroglobulin).
- Follicular cells produce thyroid hormones (T3 and T4) in response to thyroid-stimulating hormone (TSH) from the anterior pituitary.
- Parafollicular (C) cells, found between follicles, secrete calcitonin to regulate blood calcium.
Thyroid Hormone Synthesis and Effects
- Follicular cells secrete thyroglobulin into the follicle lumen.
- Iodide from the blood is converted to iodine, then binds tyrosines in thyroglobulin to form T3 and T4.
- Iodinated thyroglobulin is stored, later hydrolyzed to release T3 and T4 into the bloodstream.
- T3 (triiodothyronine) and T4 (thyroxine) increase metabolic rate, oxygen use, heart rate, and are vital for growth.
- Hyperthyroidism (excess TH) causes weight loss, hyperactivity, heat intolerance, and goiter.
- Hypothyroidism (deficient TH) results in weight gain, fatigue, cold sensitivity, and possible goiter.
Regulation of Thyroid Function
- TSH from the anterior pituitary stimulates follicular cells to produce T3/T4.
- TSH itself is regulated by thyrotropin-releasing hormone (TRH) from the hypothalamus.
- Thyroid disorders may arise from dysfunction in the thyroid, pituitary, or hypothalamus.
Calcitonin and Calcium Regulation
- Calcitonin, from parafollicular (C) cells, is released in response to high blood calcium (hypercalcemia).
- Calcitonin lowers blood calcium by increasing renal excretion, reducing intestinal absorption, and promoting calcium deposition in bones (stimulating osteoblasts, inhibiting osteoclasts).
- Calcitonin and parathyroid hormone (PTH) are antagonistic hormones.
Parathyroid Gland and Parathyroid Hormone
- Parathyroid glands (usually four) are located on the posterior surface of the thyroid.
- Chief (principal) cells of the parathyroid secrete PTH in response to low blood calcium (hypocalcemia).
- PTH increases blood calcium by stimulating bone resorption (osteoclasts), increasing renal reabsorption, and promoting intestinal absorption via stimulating calcitriol synthesis in kidneys.
- Excess PTH (hyperparathyroidism) can cause bone fractures, kidney stones, and cardiac issues; deficiency (hypoparathyroidism) may lead to neuromuscular symptoms.
Calcitriol and Vitamin D
- Calcitriol (active vitamin D3) is synthesized in the kidneys under PTH stimulation.
- It increases intestinal absorption of calcium, requiring dietary or skin-synthesized vitamin D3.
Key Terms & Definitions
- Thyroid follicle — Spherical functional units of the thyroid containing colloid and lined with follicular cells.
- Follicular cells — Thyroid cells that produce thyroid hormones.
- Parafollicular (C) cells — Cells between thyroid follicles that secrete calcitonin.
- Thyroglobulin (colloid) — Glycoprotein stored in follicles, precursor to T3 and T4.
- Thyroid hormones (T3, T4) — Regulate metabolism and are critical for growth.
- Thyroid-stimulating hormone (TSH) — Pituitary hormone that stimulates the thyroid.
- Calcitonin — Lowers blood calcium; secreted by thyroid C cells.
- Parathyroid hormone (PTH) — Raises blood calcium; secreted by parathyroid chief cells.
- Calcitriol — Active vitamin D3; increases intestinal calcium absorption.
Action Items / Next Steps
- Watch the recommended thyroid hormone and PTH overview videos for further understanding.
- Review the anatomy and histology images/models of the thyroid and parathyroid glands.
- Ensure you can explain the feedback regulation of thyroid and parathyroid hormones.