Thyroid Gland Structure

Overview of the Thyroid Gland

The thyroid gland is the largest pure endocrine gland in the human body, located on the anterior aspect of the trachea, just inferior to the larynx. It plays a crucial role in regulating metabolism and calcium homeostasis.

• Shape: Butterfly-shaped, consisting of two lateral lobes connected by an isthmus (median tissue mass).

• Follicles: Spherical structures filled with colloid, lined by epithelial follicular cells that produce glycoprotein thyroglobulin.

• Colloid: Thick fluid containing thyroglobulin with attached iodine, serving as the precursor for thyroid hormone synthesis.

• Parafollicular cells: Located around follicles, these cells produce the hormone calcitonin.

Thyroid Hormone (TH): Synthesis, Function, and Regulation

Thyroid Hormone Types and Properties

The thyroid hormone (TH) is the body's major metabolic hormone, affecting nearly all cells. It is produced in two forms:

• T4 (tetraiodothyronine or thyroxine): Consists of two tyrosine molecules with four bound iodine atoms.

• T3 (triiodothyronine): Consists of two tyrosine molecules with three bound iodine atoms.

• Both are classified as amine hormones but are lipid-soluble, similar to steroids.

• TH travels in the bloodstream bound to carrier proteins and enters target cells to bind to intracellular receptors in the nucleus, triggering transcription of genes for protein synthesis.

Physiological Effects of Thyroid Hormone

Thyroid hormone exerts multiple effects on the body:

• Increases basal metabolic rate and heat production (calorigenic effect).

• Regulates tissue growth and development:

  • Critical for normal skeletal and nervous system development.

  • Essential for reproductive capabilities.

• Permissive for catecholamines (epinephrine and norepinephrine):

  • Normal TH levels maintain normal numbers of adrenergic receptors in blood vessels, helping to regulate blood pressure.

Synthesis of Thyroid Hormone

Thyroid hormone synthesis involves several steps and occurs extracellularly in the colloid:

1. Thyroglobulin synthesis: Produced by follicular cells and transported into the follicle lumen.

2. Iodide uptake: Iodide (I-) is actively transported into the cell and then into the follicle lumen.

3. Iodide oxidation: Iodide is oxidized to iodine (I2).

4. Iodine attachment: Iodine is attached to tyrosine residues in thyroglobulin via peroxidase enzymes:

   • Iodine + tyrosine = Monoiodotyrosine (MIT)

   • 2 Iodines + tyrosine = Diiodotyrosine (DIT)

5. Coupling: MIT and DIT are coupled to form T3 and T4:

6. Endocytosis: Colloid with thyroglobulin is brought into follicular cells via endocytosis and enzymatic cleavage releases T3 and T4.

7. Secretion: T4 is secreted, and at target tissues, T4 is converted to T3 (which is 10 times more active).

Transport and Regulation of Thyroid Hormone

• T3 and T4 are transported by thyroxine-binding globulins (TBGs).

• Both hormones bind to target receptors, but T3 is more potent.

• Target tissues contain enzymes that convert T4 to T3 by removing an iodine atom.

• TH release is regulated by negative feedback:

  • Falling TH levels stimulate release of thyroid-stimulating hormone (TSH).

  • Rising TH levels (and other factors such as GHIH, dopamine, cortisol) inhibit TSH release.

  • Thyrotropin-releasing hormone (TRH) from the hypothalamus can override negative feedback during pregnancy or cold exposure, especially in infants.

Homeostatic Imbalances of Thyroid Function

Hypothyroidism

Hypothyroidism is the undersecretion of thyroid hormone (TH) and can lead to various symptoms and conditions:

• Edema: Swelling due to fluid accumulation.

• Symptoms: Low metabolic rate, thick/dry skin, puffy eyes, feeling chilled, constipation, mental sluggishness, lethargy.

• Goiter: Enlargement of the thyroid gland due to iodine deficiency.

  • Lack of iodine decreases TH levels, increasing TSH secretion, which stimulates the thyroid to enlarge and produce more unusable thyroglobulin.

• Congenital hypothyroidism: Usually caused by poor development of the thyroid gland in early childhood.

  • TH is critical for normal growth and brain development.

  • Symptoms may include weak cry, poor feeding, constipation, or prolonged jaundice.

  • TH replacement therapy is crucial and typically lifelong.

Hyperthyroidism (Graves' Disease)

Graves' disease is the most common form of hyperthyroidism, an autoimmune disorder:

• Immune system produces abnormal antibodies that mimic TSH, stimulating the thyroid to grow and secrete excess TH.

• Symptoms: Elevated metabolic rate, sweating, rapid/irregular heartbeats, nervousness, weight loss despite adequate food intake.

• Exophthalmos: Protrusion of the eyes due to edematous and fibrous tissue behind the eyes.

• Treatments: Surgical removal of the thyroid or use of radioactive iodine to destroy overactive thyroid tissue.

Parafollicular Cells and Calcitonin

Calcitonin Function

Calcitonin is produced by parafollicular (C) cells in response to elevated blood calcium levels. It acts as an antagonist to parathyroid hormone (PTH).

• Physiological role in humans at normal levels is unclear, but at higher doses, it inhibits osteoclast activity and prevents calcium release from bone.

• Stimulates calcium uptake and incorporation into bone matrix.

Parathyroid Glands: Structure and Function

Overview of Parathyroid Glands

The parathyroid glands are small, yellow-brown glands usually found embedded in the posterior aspect of the thyroid gland. Their number and location can vary.

• Contain oxyphil cells (function unclear) and parathyroid cells that secrete parathyroid hormone (PTH).

• PTH is the key regulator of calcium homeostasis.

• Low blood calcium stimulates PTH release; high calcium inhibits it.

• Main target organs: bones, kidneys, and small intestine.

Actions of Parathyroid Hormone (PTH)

• Stimulates osteoclasts to digest bone matrix and release calcium into the blood.

• Promotes reabsorption of calcium and excretion of phosphate by the kidneys.

• Activates vitamin D in the kidneys, increasing calcium absorption by the intestinal mucosa.

Homeostatic Imbalances of Parathyroid Function

Hyperparathyroidism

Hyperparathyroidism is usually caused by a parathyroid gland tumor, leading to excess PTH secretion.

• Calcium leaches from bones, causing them to soften and deform (osteitis fibrosa cystica).

• Elevated blood calcium (hypercalcemia) depresses the nervous system and contributes to kidney stone formation.

Hypoparathyroidism

Hypoparathyroidism (insufficient PTH) may follow parathyroid gland trauma or removal.

• Results in hypocalcemia (low blood calcium), increasing neuronal excitability.

• Symptoms: Tingling sensations, tetany (muscle twitching), and convulsions.