Thyroid and Parathyroid Glands


  • Thyroid Gland Anatomy

    • The thyroid gland is located in the anterior neck, inferior to the larynx and anterior to the trachea, composed of two lobes connected by an isthmus. Under microscopic examination, the thyroid gland consists of thyroid follicles filled with colloid, which stores thyroglobulin, the precursor to thyroid hormones and holds a high concentration of iodine atoms. The following cell types are present:

      • Follicular Cells: Produce thyroid hormones (T3 and T4).

      • Parafollicular Cells (C Cells): Produce calcitonin, which plays a role in calcium homeostasis.

    • Requirements for producing thyroid hormone include iodine, tyrosine, and the presence of specific enzymes.

    • Thyroid receptors, often found in nearly every cell of the body, bind to T3 and T4 to regulate metabolic processes.

    • Thyroid hormone consists of an amino acid core

  • Thyroid Hormones (T3 and T4) and Calcitonin

    • Thyroid Hormone Release:

      • Stimulus for Release: Thyroid-stimulating hormone (TSH) from the anterior pituitary.

      • Target Tissue: Most body tissues.

      • Functional Effect: Increases metabolic rate, promotes growth and development, and regulates various physiological functions.

    • Calcitonin Release:

      • Stimulus for Release: Increased blood calcium levels.

      • Target Tissue: Bone (osteoclasts) and kidneys.

      • Functional Effect: Lowers blood calcium levels by inhibiting osteoclast activity and promoting calcium excretion in the urine.

  1. Thyroid Hormone Effects

  • Effects of thyroid hormone: Almost every cell in body contains thyroid hormone

    receptors; makes their effects widespread; three main categories of effects are as follows:


  • Regulation of metabolic rate and thermoregulation – thyroid hormones set basal

    metabolic rate (amount of energy required by body at rest) by increasing rate of ATP

    consumption, increasing gluconeogenesis, and by initiating energy-requiring reactions in

    these same target cells; heat is generated, which is critical for core body temperature

    homeostasis


  • Thyroid Hormone Effects

     Effects of thyroid hormone (continued):


    • o Promotion of growth and development – thyroid hormones are required for normal bone

      growth, muscle growth, and nervous system development


    • Synergism with sympathetic nervous system –increases in thyroid hormone levels act on

      target cells of sympathetic nervous system and increase (up-regulate) receptors for

      sympathetic neurotransmitters; affects regulation of blood pressure, heart rate, and other

      sympathetic activities



  • Calcitonin – produced and secreted by parafollicular cells; released when calcium ion level in

    blood increases above normal:


    • Primary target is osteoclast cells in bone; osteoclast activity is inhibited by presence of

      calcitonin; allows osteoblast activity


    • Unopposed osteoblast activity decreases blood calcium ion levels as these ions

      are incorporated into bone matrix

Gross Anatomy of Parathyroid

  • Location: Found on the posterior aspect of the thyroid gland.

  • Number: Normally, there are 3 to 5 glands per individual.

Microscopic Anatomy

  • Chief Cells: These cells are the primary functional units of the parathyroid glands. They are responsible for synthesizing and secreting PTH. The chief cells appear as moderately sized cuboidal cells in histological sections, featuring a central nucleus and a cytoplasm rich in secretory granules containing PTH.

  • Oxyphil Cells: These are larger cells whose function is less understood. They appear after puberty and may play a role in the health of the gland but do not produce PTH.

Parathyroid Hormone (PTH)

PTH is the principal hormone secreted by the parathyroid glands, critical for maintaining calcium ion homeostasis in the body.

Stimulus for Release

  • Declining Blood Calcium Levels: The secretion of PTH is primarily triggered by a decrease in blood calcium ion concentration. This decline is detected by calcium-sensing receptors on the surface of chief cells in the parathyroid glands.

Target Tissues

  • Bones: PTH acts on osteoclasts, stimulating their activity which increases the release of calcium ions into the bloodstream from the bone matrix.

  • Kidneys: PTH promotes calcium reabsorption from the urine, reducing calcium loss. It also stimulates the conversion of vitamin D into calcitriol, which enhances intestinal absorption of calcium.

  • Small Intestine: Through the action of activated vitamin D (calcitriol), PTH indirectly increases the intestinal absorption of dietary calcium.

Functional Effects of PTH

  • Increased Bone Resorption: PTH enhances osteoclast activity leading to an increase in the release of calcium and phosphate into the bloodstream. This effect supports the elevation of blood calcium levels but can also lead to bone loss if PTH is chronically elevated.

  • Increased Renal Reabsorption: PTH acts on the kidneys to promote the reabsorption of calcium ions and, at the same time, encourage the excretion of phosphate, which plays a significant role in maintaining appropriate calcium-phosphate balance in the body.

  • Activation of Vitamin D: By stimulating the conversion of vitamin D to its active form, calcitriol, PTH enhances the absorption of calcium from the diet, contributing to the overall increase in blood calcium levels.

The interplay between PTH and calcitonin (which is secreted by the thyroid gland and has opposing effects) is crucial in tightly regulating calcium levels within a normal physiological range, ensuring proper biochemical and physiological functions throughout the body.