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Anatomy and Physiology

17.4 The Thyroid Gland

Anatomy and Physiology17.4 The Thyroid Gland
  1. Preface
  2. Unit 1: Levels of Organization
    1. 1 An Introduction to the Human Body
      1. Introduction
      2. 1.1 Overview of Anatomy and Physiology
      3. 1.2 Structural Organization of the Human Body
      4. 1.3 Functions of Human Life
      5. 1.4 Requirements for Human Life
      6. 1.5 Homeostasis
      7. 1.6 Anatomical Terminology
      8. 1.7 Medical Imaging
      9. Key Terms
      10. Chapter Review
      11. Interactive Link Questions
      12. Review Questions
      13. Critical Thinking Questions
    2. 2 The Chemical Level of Organization
      1. Introduction
      2. 2.1 Elements and Atoms: The Building Blocks of Matter
      3. 2.2 Chemical Bonds
      4. 2.3 Chemical Reactions
      5. 2.4 Inorganic Compounds Essential to Human Functioning
      6. 2.5 Organic Compounds Essential to Human Functioning
      7. Key Terms
      8. Chapter Review
      9. Interactive Link Questions
      10. Review Questions
      11. Critical Thinking Questions
    3. 3 The Cellular Level of Organization
      1. Introduction
      2. 3.1 The Cell Membrane
      3. 3.2 The Cytoplasm and Cellular Organelles
      4. 3.3 The Nucleus and DNA Replication
      5. 3.4 Protein Synthesis
      6. 3.5 Cell Growth and Division
      7. 3.6 Cellular Differentiation
      8. Key Terms
      9. Chapter Review
      10. Interactive Link Questions
      11. Review Questions
      12. Critical Thinking Questions
    4. 4 The Tissue Level of Organization
      1. Introduction
      2. 4.1 Types of Tissues
      3. 4.2 Epithelial Tissue
      4. 4.3 Connective Tissue Supports and Protects
      5. 4.4 Muscle Tissue and Motion
      6. 4.5 Nervous Tissue Mediates Perception and Response
      7. 4.6 Tissue Injury and Aging
      8. Key Terms
      9. Chapter Review
      10. Interactive Link Questions
      11. Review Questions
      12. Critical Thinking Questions
  3. Unit 2: Support and Movement
    1. 5 The Integumentary System
      1. Introduction
      2. 5.1 Layers of the Skin
      3. 5.2 Accessory Structures of the Skin
      4. 5.3 Functions of the Integumentary System
      5. 5.4 Diseases, Disorders, and Injuries of the Integumentary System
      6. Key Terms
      7. Chapter Review
      8. Interactive Link Questions
      9. Review Questions
      10. Critical Thinking Questions
    2. 6 Bone Tissue and the Skeletal System
      1. Introduction
      2. 6.1 The Functions of the Skeletal System
      3. 6.2 Bone Classification
      4. 6.3 Bone Structure
      5. 6.4 Bone Formation and Development
      6. 6.5 Fractures: Bone Repair
      7. 6.6 Exercise, Nutrition, Hormones, and Bone Tissue
      8. 6.7 Calcium Homeostasis: Interactions of the Skeletal System and Other Organ Systems
      9. Key Terms
      10. Chapter Review
      11. Review Questions
      12. Critical Thinking Questions
    3. 7 Axial Skeleton
      1. Introduction
      2. 7.1 Divisions of the Skeletal System
      3. 7.2 The Skull
      4. 7.3 The Vertebral Column
      5. 7.4 The Thoracic Cage
      6. 7.5 Embryonic Development of the Axial Skeleton
      7. Key Terms
      8. Chapter Review
      9. Interactive Link Questions
      10. Review Questions
      11. Critical Thinking Questions
    4. 8 The Appendicular Skeleton
      1. Introduction
      2. 8.1 The Pectoral Girdle
      3. 8.2 Bones of the Upper Limb
      4. 8.3 The Pelvic Girdle and Pelvis
      5. 8.4 Bones of the Lower Limb
      6. 8.5 Development of the Appendicular Skeleton
      7. Key Terms
      8. Chapter Review
      9. Interactive Link Questions
      10. Review Questions
      11. Critical Thinking Questions
    5. 9 Joints
      1. Introduction
      2. 9.1 Classification of Joints
      3. 9.2 Fibrous Joints
      4. 9.3 Cartilaginous Joints
      5. 9.4 Synovial Joints
      6. 9.5 Types of Body Movements
      7. 9.6 Anatomy of Selected Synovial Joints
      8. 9.7 Development of Joints
      9. Key Terms
      10. Chapter Review
      11. Interactive Link Questions
      12. Review Questions
      13. Critical Thinking Questions
    6. 10 Muscle Tissue
      1. Introduction
      2. 10.1 Overview of Muscle Tissues
      3. 10.2 Skeletal Muscle
      4. 10.3 Muscle Fiber Contraction and Relaxation
      5. 10.4 Nervous System Control of Muscle Tension
      6. 10.5 Types of Muscle Fibers
      7. 10.6 Exercise and Muscle Performance
      8. 10.7 Cardiac Muscle Tissue
      9. 10.8 Smooth Muscle
      10. 10.9 Development and Regeneration of Muscle Tissue
      11. Key Terms
      12. Chapter Review
      13. Interactive Link Questions
      14. Review Questions
      15. Critical Thinking Questions
    7. 11 The Muscular System
      1. Introduction
      2. 11.1 Interactions of Skeletal Muscles, Their Fascicle Arrangement, and Their Lever Systems
      3. 11.2 Naming Skeletal Muscles
      4. 11.3 Axial Muscles of the Head, Neck, and Back
      5. 11.4 Axial Muscles of the Abdominal Wall, and Thorax
      6. 11.5 Muscles of the Pectoral Girdle and Upper Limbs
      7. 11.6 Appendicular Muscles of the Pelvic Girdle and Lower Limbs
      8. Key Terms
      9. Chapter Review
      10. Review Questions
      11. Critical Thinking Questions
  4. Unit 3: Regulation, Integration, and Control
    1. 12 The Nervous System and Nervous Tissue
      1. Introduction
      2. 12.1 Basic Structure and Function of the Nervous System
      3. 12.2 Nervous Tissue
      4. 12.3 The Function of Nervous Tissue
      5. 12.4 The Action Potential
      6. 12.5 Communication Between Neurons
      7. Key Terms
      8. Chapter Review
      9. Interactive Link Questions
      10. Review Questions
      11. Critical Thinking Questions
    2. 13 Anatomy of the Nervous System
      1. Introduction
      2. 13.1 The Embryologic Perspective
      3. 13.2 The Central Nervous System
      4. 13.3 Circulation and the Central Nervous System
      5. 13.4 The Peripheral Nervous System
      6. Key Terms
      7. Chapter Review
      8. Interactive Link Questions
      9. Review Questions
      10. Critical Thinking Questions
    3. 14 The Somatic Nervous System
      1. Introduction
      2. 14.1 Sensory Perception
      3. 14.2 Central Processing
      4. 14.3 Motor Responses
      5. Key Terms
      6. Chapter Review
      7. Interactive Link Questions
      8. Review Questions
      9. Critical Thinking Questions
    4. 15 The Autonomic Nervous System
      1. Introduction
      2. 15.1 Divisions of the Autonomic Nervous System
      3. 15.2 Autonomic Reflexes and Homeostasis
      4. 15.3 Central Control
      5. 15.4 Drugs that Affect the Autonomic System
      6. Key Terms
      7. Chapter Review
      8. Interactive Link Questions
      9. Review Questions
      10. Critical Thinking Questions
    5. 16 The Neurological Exam
      1. Introduction
      2. 16.1 Overview of the Neurological Exam
      3. 16.2 The Mental Status Exam
      4. 16.3 The Cranial Nerve Exam
      5. 16.4 The Sensory and Motor Exams
      6. 16.5 The Coordination and Gait Exams
      7. Key Terms
      8. Chapter Review
      9. Interactive Link Questions
      10. Review Questions
      11. Critical Thinking Questions
    6. 17 The Endocrine System
      1. Introduction
      2. 17.1 An Overview of the Endocrine System
      3. 17.2 Hormones
      4. 17.3 The Pituitary Gland and Hypothalamus
      5. 17.4 The Thyroid Gland
      6. 17.5 The Parathyroid Glands
      7. 17.6 The Adrenal Glands
      8. 17.7 The Pineal Gland
      9. 17.8 Gonadal and Placental Hormones
      10. 17.9 The Endocrine Pancreas
      11. 17.10 Organs with Secondary Endocrine Functions
      12. 17.11 Development and Aging of the Endocrine System
      13. Key Terms
      14. Chapter Review
      15. Interactive Link Questions
      16. Review Questions
      17. Critical Thinking Questions
  5. Unit 4: Fluids and Transport
    1. 18 The Cardiovascular System: Blood
      1. Introduction
      2. 18.1 An Overview of Blood
      3. 18.2 Production of the Formed Elements
      4. 18.3 Erythrocytes
      5. 18.4 Leukocytes and Platelets
      6. 18.5 Hemostasis
      7. 18.6 Blood Typing
      8. Key Terms
      9. Chapter Review
      10. Interactive Link Questions
      11. Review Questions
      12. Critical Thinking Questions
    2. 19 The Cardiovascular System: The Heart
      1. Introduction
      2. 19.1 Heart Anatomy
      3. 19.2 Cardiac Muscle and Electrical Activity
      4. 19.3 Cardiac Cycle
      5. 19.4 Cardiac Physiology
      6. 19.5 Development of the Heart
      7. Key Terms
      8. Chapter Review
      9. Interactive Link Questions
      10. Review Questions
      11. Critical Thinking Questions
    3. 20 The Cardiovascular System: Blood Vessels and Circulation
      1. Introduction
      2. 20.1 Structure and Function of Blood Vessels
      3. 20.2 Blood Flow, Blood Pressure, and Resistance
      4. 20.3 Capillary Exchange
      5. 20.4 Homeostatic Regulation of the Vascular System
      6. 20.5 Circulatory Pathways
      7. 20.6 Development of Blood Vessels and Fetal Circulation
      8. Key Terms
      9. Chapter Review
      10. Interactive Link Questions
      11. Review Questions
      12. Critical Thinking Questions
    4. 21 The Lymphatic and Immune System
      1. Introduction
      2. 21.1 Anatomy of the Lymphatic and Immune Systems
      3. 21.2 Barrier Defenses and the Innate Immune Response
      4. 21.3 The Adaptive Immune Response: T lymphocytes and Their Functional Types
      5. 21.4 The Adaptive Immune Response: B-lymphocytes and Antibodies
      6. 21.5 The Immune Response against Pathogens
      7. 21.6 Diseases Associated with Depressed or Overactive Immune Responses
      8. 21.7 Transplantation and Cancer Immunology
      9. Key Terms
      10. Chapter Review
      11. Interactive Link Questions
      12. Review Questions
      13. Critical Thinking Questions
  6. Unit 5: Energy, Maintenance, and Environmental Exchange
    1. 22 The Respiratory System
      1. Introduction
      2. 22.1 Organs and Structures of the Respiratory System
      3. 22.2 The Lungs
      4. 22.3 The Process of Breathing
      5. 22.4 Gas Exchange
      6. 22.5 Transport of Gases
      7. 22.6 Modifications in Respiratory Functions
      8. 22.7 Embryonic Development of the Respiratory System
      9. Key Terms
      10. Chapter Review
      11. Interactive Link Questions
      12. Review Questions
      13. Critical Thinking Questions
    2. 23 The Digestive System
      1. Introduction
      2. 23.1 Overview of the Digestive System
      3. 23.2 Digestive System Processes and Regulation
      4. 23.3 The Mouth, Pharynx, and Esophagus
      5. 23.4 The Stomach
      6. 23.5 The Small and Large Intestines
      7. 23.6 Accessory Organs in Digestion: The Liver, Pancreas, and Gallbladder
      8. 23.7 Chemical Digestion and Absorption: A Closer Look
      9. Key Terms
      10. Chapter Review
      11. Interactive Link Questions
      12. Review Questions
      13. Critical Thinking Questions
    3. 24 Metabolism and Nutrition
      1. Introduction
      2. 24.1 Overview of Metabolic Reactions
      3. 24.2 Carbohydrate Metabolism
      4. 24.3 Lipid Metabolism
      5. 24.4 Protein Metabolism
      6. 24.5 Metabolic States of the Body
      7. 24.6 Energy and Heat Balance
      8. 24.7 Nutrition and Diet
      9. Key Terms
      10. Chapter Review
      11. Review Questions
      12. Critical Thinking Questions
    4. 25 The Urinary System
      1. Introduction
      2. 25.1 Physical Characteristics of Urine
      3. 25.2 Gross Anatomy of Urine Transport
      4. 25.3 Gross Anatomy of the Kidney
      5. 25.4 Microscopic Anatomy of the Kidney
      6. 25.5 Physiology of Urine Formation
      7. 25.6 Tubular Reabsorption
      8. 25.7 Regulation of Renal Blood Flow
      9. 25.8 Endocrine Regulation of Kidney Function
      10. 25.9 Regulation of Fluid Volume and Composition
      11. 25.10 The Urinary System and Homeostasis
      12. Key Terms
      13. Chapter Review
      14. Review Questions
      15. Critical Thinking Questions
    5. 26 Fluid, Electrolyte, and Acid-Base Balance
      1. Introduction
      2. 26.1 Body Fluids and Fluid Compartments
      3. 26.2 Water Balance
      4. 26.3 Electrolyte Balance
      5. 26.4 Acid-Base Balance
      6. 26.5 Disorders of Acid-Base Balance
      7. Key Terms
      8. Chapter Review
      9. Interactive Link Questions
      10. Review Questions
      11. Critical Thinking Questions
  7. Unit 6: Human Development and the Continuity of Life
    1. 27 The Reproductive System
      1. Introduction
      2. 27.1 Anatomy and Physiology of the Male Reproductive System
      3. 27.2 Anatomy and Physiology of the Female Reproductive System
      4. 27.3 Development of the Male and Female Reproductive Systems
      5. Key Terms
      6. Chapter Review
      7. Interactive Link Questions
      8. Review Questions
      9. Critical Thinking Questions
    2. 28 Development and Inheritance
      1. Introduction
      2. 28.1 Fertilization
      3. 28.2 Embryonic Development
      4. 28.3 Fetal Development
      5. 28.4 Maternal Changes During Pregnancy, Labor, and Birth
      6. 28.5 Adjustments of the Infant at Birth and Postnatal Stages
      7. 28.6 Lactation
      8. 28.7 Patterns of Inheritance
      9. Key Terms
      10. Chapter Review
      11. Interactive Link Questions
      12. Review Questions
      13. Critical Thinking Questions
  8. References
  9. Index
By the end of this section, you will be able to:
  • Describe the location and anatomy of the thyroid gland
  • Discuss the synthesis of triiodothyronine and thyroxine
  • Explain the role of thyroid hormones in the regulation of basal metabolism
  • Identify the hormone produced by the parafollicular cells of the thyroid

A butterfly-shaped organ, the thyroid gland is located anterior to the trachea, just inferior to the larynx (Figure 17.12). The medial region, called the isthmus, is flanked by wing-shaped left and right lobes. Each of the thyroid lobes are embedded with parathyroid glands, primarily on their posterior surfaces. The tissue of the thyroid gland is composed mostly of thyroid follicles. The follicles are made up of a central cavity filled with a sticky fluid called colloid. Surrounded by a wall of epithelial follicle cells, the colloid is the center of thyroid hormone production, and that production is dependent on the hormones’ essential and unique component: iodine.

Part A of this figure is a diagram of the anterior view of the thyroid gland. The thyroid gland is a butterfly-shaped gland wrapping around the trachea. It narrows at its center, just under the thyroid cartilage of the larynx. This narrow area is called the isthmus of the thyroid. Two large arteries, the common carotid arteries, run parallel to the trachea on the outer border of the thyroid. A small artery enters the superior edge of the thyroid, near the isthmus, and branches throughout the two “wings” of the thyroid. Part B of this figure is a posterior view of the thyroid. The posterior view shows that the thyroid does not completely wrap around the posterior of the trachea. The posterior sides of the thyroid wings can be seen protruding from under the cricoid cartilage of the larynx. The posterior sides of the thyroid “wings” each contain two small, disc-shaped parathyroid glands embedded in the thyroid tissue. Within each wing, one disc is located superior to the other. These are labeled the left and right parathyroid glands. Just under the inferior parathyroid glands are two arteries that bring blood to the thyroid from the left and right subclavian arteries. Part C of this figure is a micrograph of thyroid tissue. The thyroid follicle cells are cuboidal epithelial cells. These cells form a ring around irregular-shaped cavities called follicles. The follicles contain light colored colloid. A larger parafollicular cell is embedded between two of the follicular cells near the edge of a follicle.
Figure 17.12 Thyroid Gland The thyroid gland is located in the neck where it wraps around the trachea. (a) Anterior view of the thyroid gland. (b) Posterior view of the thyroid gland. (c) The glandular tissue is composed primarily of thyroid follicles. The larger parafollicular cells often appear within the matrix of follicle cells. LM × 1332. (Micrograph provided by the Regents of University of Michigan Medical School © 2012)

Synthesis and Release of Thyroid Hormones

Hormones are produced in the colloid when atoms of the mineral iodine attach to a glycoprotein, called thyroglobulin, that is secreted into the colloid by the follicle cells. The following steps outline the hormones’ assembly:

  1. Binding of TSH to its receptors in the follicle cells of the thyroid gland causes the cells to actively transport iodide ions (I) across their cell membrane, from the bloodstream into the cytosol. As a result, the concentration of iodide ions “trapped” in the follicular cells is many times higher than the concentration in the bloodstream.
  2. Iodide ions then move to the lumen of the follicle cells that border the colloid. There, the ions undergo oxidation (their negatively charged electrons are removed). The oxidation of two iodide ions (2 I) results in iodine (I2), which passes through the follicle cell membrane into the colloid.
  3. In the colloid, peroxidase enzymes link the iodine to the tyrosine amino acids in thyroglobulin to produce two intermediaries: a tyrosine attached to one iodine and a tyrosine attached to two iodines. When one of each of these intermediaries is linked by covalent bonds, the resulting compound is triiodothyronine (T3), a thyroid hormone with three iodines. Much more commonly, two copies of the second intermediary bond, forming tetraiodothyronine, also known as thyroxine (T4), a thyroid hormone with four iodines.

These hormones remain in the colloid center of the thyroid follicles until TSH stimulates endocytosis of colloid back into the follicle cells. There, lysosomal enzymes break apart the thyroglobulin colloid, releasing free T3 and T4, which diffuse across the follicle cell membrane and enter the bloodstream.

In the bloodstream, less than one percent of the circulating T3 and T4 remains unbound. This free T3 and T4 can cross the lipid bilayer of cell membranes and be taken up by cells. The remaining 99 percent of circulating T3 and T4 is bound to specialized transport proteins called thyroxine-binding globulins (TBGs), to albumin, or to other plasma proteins. This “packaging” prevents their free diffusion into body cells. When blood levels of T3 and T4 begin to decline, bound T3 and T4 are released from these plasma proteins and readily cross the membrane of target cells. T3 is more potent than T4, and many cells convert T4 to T3 through the removal of an iodine atom.

Regulation of TH Synthesis

The release of T3 and T4 from the thyroid gland is regulated by thyroid-stimulating hormone (TSH). As shown in Figure 17.13, low blood levels of T3 and T4 stimulate the release of thyrotropin-releasing hormone (TRH) from the hypothalamus, which triggers secretion of TSH from the anterior pituitary. In turn, TSH stimulates the thyroid gland to secrete T3 and T4. The levels of TRH, TSH, T3, and T4 are regulated by a negative feedback system in which increasing levels of T3 and T4 decrease the production and secretion of TSH.

This diagram illustrates a negative feedback loop. It shows the general steps of a negative feedback loop at the center (imbalance, hormone release, correction, and negative feedback) using the example of the hormone cascade that regulates metabolic rate. The hypothalamus releases TRH in response to low metabolic rate and or low T three and T four concentrations in the blood (imbalance). This triggers TSH release by the pituitary (hormone release). The TSH travels to the thyroid where it triggers T three and T four release by the thyroid cells. T three and T four increase basal metabolic rate of the body cells and cause a rise in body temperature (the calorigenic effect). T three and T four then feed back to the hypothalamus and inhibits TRH and TSH release. If metabolic rate is high and or T three and T four concentrations are low, then the hypothalamus stops releasing TRH (negative feedback). As a result, the anterior pituitary will not release TSH and no T three or T four will be produced by the thyroid.
Figure 17.13 Classic Negative Feedback Loop A classic negative feedback loop controls the regulation of thyroid hormone levels.

Functions of Thyroid Hormones

The thyroid hormones, T3 and T4, are often referred to as metabolic hormones because their levels influence the body’s basal metabolic rate, the amount of energy used by the body at rest. When T3 and T4 bind to intracellular receptors located on the mitochondria, they cause an increase in nutrient breakdown and the use of oxygen to produce ATP. In addition, T3 and T4 initiate the transcription of genes involved in glucose oxidation. Although these mechanisms prompt cells to produce more ATP, the process is inefficient, and an abnormally increased level of heat is released as a byproduct of these reactions. This so-called calorigenic effect (calor- = “heat”) raises body temperature.

Adequate levels of thyroid hormones are also required for protein synthesis and for fetal and childhood tissue development and growth. They are especially critical for normal development of the nervous system both in utero and in early childhood, and they continue to support neurological function in adults. As noted earlier, these thyroid hormones have a complex interrelationship with reproductive hormones, and deficiencies can influence libido, fertility, and other aspects of reproductive function. Finally, thyroid hormones increase the body’s sensitivity to catecholamines (epinephrine and norepinephrine) from the adrenal medulla by upregulation of receptors in the blood vessels. When levels of T3 and T4 hormones are excessive, this effect accelerates the heart rate, strengthens the heartbeat, and increases blood pressure. Because thyroid hormones regulate metabolism, heat production, protein synthesis, and many other body functions, thyroid disorders can have severe and widespread consequences.

Disorders of the...

Endocrine System: Iodine Deficiency, Hypothyroidism, and Hyperthyroidism

As discussed above, dietary iodine is required for the synthesis of T3 and T4. But for much of the world’s population, foods do not provide adequate levels of this mineral, because the amount varies according to the level in the soil in which the food was grown, as well as the irrigation and fertilizers used. Marine fish and shrimp tend to have high levels because they concentrate iodine from seawater, but many people in landlocked regions lack access to seafood. Thus, the primary source of dietary iodine in many countries is iodized salt. Fortification of salt with iodine began in the United States in 1924, and international efforts to iodize salt in the world’s poorest nations continue today.

Dietary iodine deficiency can result in the impaired ability to synthesize T3 and T4, leading to a variety of severe disorders. When T3 and T4 cannot be produced, TSH is secreted in increasing amounts. As a result of this hyperstimulation, thyroglobulin accumulates in the thyroid gland follicles, increasing their deposits of colloid. The accumulation of colloid increases the overall size of the thyroid gland, a condition called a goiter (Figure 17.14). A goiter is only a visible indication of the deficiency. Other iodine deficiency disorders include impaired growth and development, decreased fertility, and prenatal and infant death. Moreover, iodine deficiency is the primary cause of preventable mental retardation worldwide. Neonatal hypothyroidism (cretinism) is characterized by cognitive deficits, short stature, and sometimes deafness and muteness in children and adults born to mothers who were iodine-deficient during pregnancy.

This photo shows a woman with a goiter, which is an extreme, irregular swelling on the anterior side of the neck.
Figure 17.14 Goiter (credit: “Almazi”/Wikimedia Commons)

In areas of the world with access to iodized salt, dietary deficiency is rare. Instead, inflammation of the thyroid gland is the more common cause of low blood levels of thyroid hormones. Called hypothyroidism, the condition is characterized by a low metabolic rate, weight gain, cold extremities, constipation, reduced libido, menstrual irregularities, and reduced mental activity. In contrast, hyperthyroidism—an abnormally elevated blood level of thyroid hormones—is often caused by a pituitary or thyroid tumor. In Graves’ disease, the hyperthyroid state results from an autoimmune reaction in which antibodies overstimulate the follicle cells of the thyroid gland. Hyperthyroidism can lead to an increased metabolic rate, excessive body heat and sweating, diarrhea, weight loss, tremors, and increased heart rate. The person’s eyes may bulge (called exophthalmos) as antibodies produce inflammation in the soft tissues of the orbits. The person may also develop a goiter.

Calcitonin

The thyroid gland also secretes a hormone called calcitonin that is produced by the parafollicular cells (also called C cells) that stud the tissue between distinct follicles. Calcitonin is released in response to a rise in blood calcium levels. It appears to have a function in decreasing blood calcium concentrations by:

  • Inhibiting the activity of osteoclasts, bone cells that release calcium into the circulation by degrading bone matrix
  • Increasing osteoblastic activity
  • Decreasing calcium absorption in the intestines
  • Increasing calcium loss in the urine

However, these functions are usually not significant in maintaining calcium homeostasis, so the importance of calcitonin is not entirely understood. Pharmaceutical preparations of calcitonin are sometimes prescribed to reduce osteoclast activity in people with osteoporosis and to reduce the degradation of cartilage in people with osteoarthritis. The hormones secreted by thyroid are summarized in Table 17.4.

Thyroid Hormones
Associated hormones Chemical class Effect
Thyroxine (T4), triiodothyronine (T3) Amine Stimulate basal metabolic rate
Calcitonin Peptide Reduces blood Ca2+ levels
Table 17.4

Of course, calcium is critical for many other biological processes. It is a second messenger in many signaling pathways, and is essential for muscle contraction, nerve impulse transmission, and blood clotting. Given these roles, it is not surprising that blood calcium levels are tightly regulated by the endocrine system. The organs involved in the regulation are the parathyroid glands.

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