Hormone Regulation: Calcitonin and Thyroid Hormone

Thyroid Gland

• The thyroid gland is located anterior and inferior to the larynx and consists of two lobes united by an isthmus.
• It synthesizes thyroid hormones, including thyroxine (T4), tri-iodothyronine (T3), and calcitonin.
• The gland originates from the foregut endoderm near the base of the developing tongue during early embryonic life.

Thyroid Follicles

• The thyroid gland is composed of millions of rounded epithelial thyroid follicles of variable diameter.
• Each follicle has a simple epithelium and a central lumen densely filled with gelatinous acidophilic colloid.
• The thyroid is the only endocrine gland that stores a large quantity of secretory product outside the cells, in the colloid.
• Thyroid colloid contains the large glycoprotein thyroglobulin, the precursor for active thyroid hormones.
• There is sufficient hormone in follicles to supply the body for up to 3 months without additional synthesis.

Thyroid Cells

• Follicular cells (thyrocytes) have a shape that varies from squamous to low columnar, depending on their activity.
• Their size and features vary with their activity, which is controlled by thyroid-stimulating hormone (TSH) from the anterior pituitary.
• Active glands have more follicles of low columnar epithelium, while glands with mostly squamous follicular cells are hypoactive.

Parafollicular Cells (C Cells)

• Parafollicular cells are an endocrine cell type found inside the basal lamina of the follicular epithelium or as isolated clusters between follicles.
• They are derived from the neural crest and are larger than follicular cells, staining less intensely.
• They have ultrastructural features of active polypeptide-secreting cells, with secretory granules that vary in size, morphology, and electron density.

Calcitonin

• Calcitonin is secreted by parafollicular cells (C cells) and is triggered by elevated blood Ca2+ levels.
• It inhibits osteoclast activity.

Parathyroid Glands

• The parathyroid glands are four small ovoid masses, each 3 × 6 mm in size, with a total weight of about 0.4 g.
• They are located on the back of the thyroid gland, usually embedded in the gland's capsule.
• Each gland is contained within a thin capsule, with septa extending into the gland and supporting dense elongated clusters of secretory cells.

Principal (Chief) Cells

• Principal cells are the endocrine cells of the parathyroid glands, with small polygonal cells and round nuclei.
• They have pale-staining, slightly acidophilic cytoplasm with irregularly shaped cytoplasmic granules containing parathyroid hormone (PTH).

Goiter

• Goiter is a condition caused by chronic dietary iodine deficiencies, which inhibit thyroid hormone production.
• This leads to excessive growth of thyroid follicles and enlargement of the thyroid gland.

Graves' Disease

• Graves' disease is an autoimmune disorder characterized by antibodies that produce chronic stimulation of the follicular cells and release of thyroid hormones (hyperthyroidism).
• This causes a hypermetabolic state marked by weight loss, nervousness, sweating, heat intolerance, and other features.

Hypothyroidism

• Hypothyroidism is a condition characterized by reduced thyroid hormone levels, which can be caused by local inflammation (thyroiditis) or inadequate secretion of TSH by the anterior pituitary gland.
• It is often manifested by tiredness, weight gain, intolerance of cold, and decreased ability to concentrate.