The Thyroid and Parathyroid Glands

Gross and Microscopic Anatomy of the Thyroid and Parathyroid Glands

The thyroid gland is a butterfly-shaped endocrine gland located anterior to the trachea and inferior to the larynx. It consists of two lateral lobes connected by a narrow isthmus. The parathyroid glands are typically four small, oval-shaped glands embedded in the posterior surface of the thyroid gland. Both glands play crucial roles in regulating metabolism and calcium homeostasis.

• Thyroid gland: Composed of spherical follicles filled with colloid, surrounded by follicular cells.

• Parathyroid glands: Consist mainly of chief cells and oxyphil cells, responsible for hormone secretion.

Cell Types in the Thyroid Gland

The thyroid gland contains several specialized cell types, each with distinct functions:

• Follicular cells: Produce and secrete thyroid hormones (thyroxine (T4) and triiodothyronine (T3)).

• Parafollicular cells (C cells): Secrete calcitonin, a hormone involved in calcium regulation.

Thyroid hormones are stored in the colloid within the follicles until released into the bloodstream.

Thyroid Hormone: Stimulus, Target Tissues, and Effects

Thyroid hormone release is regulated by the hypothalamic-pituitary-thyroid axis:

• Stimulus for release: Thyrotropin-releasing hormone (TRH) from the hypothalamus stimulates the anterior pituitary to release thyroid-stimulating hormone (TSH), which then stimulates the thyroid gland.

• Target tissues: Most cells in the body, especially those involved in metabolism (e.g., liver, muscle, heart).

• Effects: Increases basal metabolic rate, enhances protein synthesis, and regulates growth and development.

Example: Increased thyroid hormone levels accelerate cellular respiration and energy expenditure.

Negative Feedback Regulation of Thyroid Hormone Production

Thyroid hormone secretion is tightly controlled by a negative feedback loop:

• Elevated levels of T3 and T4 inhibit the release of TRH and TSH, reducing further hormone production.

• This feedback maintains homeostasis and prevents excessive hormone levels.

Example: If thyroid hormone levels rise above normal, TSH secretion decreases, lowering thyroid hormone output.

Parathyroid Hormone: Stimulus, Target Tissues, and Effects

The parathyroid glands secrete parathyroid hormone (PTH), which is essential for calcium balance:

• Stimulus for release: Low blood calcium levels (hypocalcemia).

• Target tissues: Bones, kidneys, and intestines.

• Effects: Increases blood calcium by stimulating osteoclast activity (bone resorption), enhancing calcium reabsorption in the kidneys, and promoting activation of vitamin D to increase intestinal calcium absorption.

Example: When blood calcium drops, PTH is released to restore normal calcium levels.