Thyroid Gland: Anatomy, Histology and Hormones

Thyroid Anatomy and Structure

• The thyroid gland is an endocrine gland located in the anterior neck, under the larynx, surrounding the trachea.
• It has two lobes (left and right) connected by a thin isthmus, and sometimes includes an additional pyramidal lobe.
• Well vascularized to enable quick release of hormones into circulation.

Histology - Microscopic Structure

• Functional units are thyroid follicles, made of a layer of follicular cells containing a gelatinous substance called colloid.
• Follicular cells produce thyroid hormones T3 (triiodothyronine) and T4 (thyroxine).
• C-cells (parafollicular) synthesize calcitonin, a hormone involved in calcium metabolism.

Thyroid Hormones – Biosynthesis and Functions

• The synthesis of T3 (triiodothyronine) and T4 (thyroxine) depends on iodine and occurs in several steps:
• Uptake of iodine from the blood by follicular cells.
• Oxidation of iodine and attachment to tyrosine to form monoiodotyrosine (MIT) and diiodotyrosine (DIT).
• MIT and DIT coupling to form T3 and T4.
• Endocytosis and release of hormones into the bloodstream.
• T4 is the predominant form (approx. 80%), while T3 is the metabolically active form.
• Functions of thyroid hormones:
• Stimulates basal metabolism and oxygen consumption.
• Regulates body temperature.
• Develops the central nervous system in the fetus and child.
• Stimulates heart rate and blood pressure.
• Supports normal growth and development.

Calcitonin

• Produced by C-cells.
• Lowers blood calcium levels by:
• Inhibiting bone resorption (decreasing osteoclast activity).
• Increasing calcium elimination through urine.

Regulation of Thyroid Function

• Hypothalamic-pituitary-thyroid axis:
• The hypothalamus secretes TRH (thyrotropin-releasing hormone).
• TRH stimulates the pituitary to release TSH (thyroid-stimulating hormone).
• TSH stimulates the thyroid to produce T3 and T4.
• T3 and T4 exert negative feedback on the hypothalamus and pituitary to regulate TSH production.

Thyroid Dysfunction

• Hyperthyroidism (Excess thyroid hormones)

• Causes:

• Graves-Basedow disease (autoimmune).

• Toxic adenoma (benign tumor).

• Toxic multinodular goiter.

• Symptoms:

• Accelerated metabolism, weight loss.

• Tachycardia, hypertension.

• Excessive sweating, heat intolerance.

• Tremors, nervousness, insomnia.

• Exophthalmos (protruding eyes) in Graves' disease.

• Hypothyroidism (Thyroid hormone deficiency)

• Causes:

• Iodine deficiency.

• Hashimoto's thyroiditis (autoimmune).

• Thyroid destruction by surgery or radiotherapy.

• Symptoms:

• Slowed metabolism, weight gain.

• Fatigue, depression, dry skin.

• Bradycardia, cold intolerance.

• Myxedema (fluid retention).

Goiter

• Enlargement of the thyroid gland.
• Can be euthyroid (normal function), hypothyroid, or hyperthyroid.