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The thyroid gland is a small, butterfly-shaped gland. It covers the anterior surface of the trachea in the neck region.

It has two lateral lobes connected by a thin tissue mass called the isthmus.

Internally, each lobe comprises many small spherical structures known as thyroid follicles, surrounded by a network of blood vessels.

Each follicle has a central cavity lined by simple cuboidal to squamous epithelial cells called follicular cells. These cells produce the glycoprotein thyroglobulin and store it in the central colloid.

Thyroglobulin acts as a precursor to triiodothyronine or T₃ and thyroxine or T₄, thyroid hormones essential for maintaining the body's metabolic rate and energy production.

Additionally, the parafollicular or C cells scattered between the follicles are responsible for the production of the hormone calcitonin, which regulates calcium metabolism in the body.

The thyroid gland is under the control of the thyroid-stimulating hormone produced by the pituitary gland.

However, rising thyroid levels send the inhibitory signal to the pituitary gland to stop secreting thyroid-stimulating hormone or TSH.