Thyroidae

Thyroidae denotes the thyroid glands in vertebrates, typically referring to the two lobes of the human thyroid. In humans, the gland consists of right and left lobes connected by an isthmus and lies in the anterior neck, just inferior to the larynx and anterior to the trachea. Microscopically, the parenchyma is organized into follicles lined by follicular cells that produce thyroid hormones and contain colloid; parafollicular C-cells secrete calcitonin.

Thyroid hormones, mainly thyroxine (T4) and triiodothyronine (T3), are synthesized from iodine within the follicular lumen and released into the bloodstream in response to thyroid-stimulating hormone (TSH) from the pituitary, itself regulated by thyrotropin-releasing hormone (TRH) from the hypothalamus. TSH stimulates hormone synthesis and follicular cell growth. T3 and T4 increase the basal metabolic rate, influence protein synthesis, and regulate development and maturation of the central nervous system, acting through nuclear receptors. Calcitonin from C-cells helps regulate calcium and phosphate homeostasis, though its systemic role is less prominent than that of thyroid hormones.

The thyroid receives arterial blood from the superior and inferior thyroid arteries and drains via an accompanying venous system; lymphatics follow the vessels to regional nodes. Embryologically, the gland develops from endoderm of the pharyngeal floor and descends to the neck, with ectopic thyroid tissue possible along the migratory path.

Clinically, thyroid dysfunction includes hypothyroidism (often autoimmune, such as Hashimoto's), hyperthyroidism (such as Graves’ disease), goiter, nodules, and cancer. Diagnosis commonly uses serum TSH and free T4, supplemented by ultrasound and fine-needle aspiration for nodules. Treatments vary by disorder, including levothyroxine for hypothyroidism and antithyroid drugs, radioiodine, or surgery for hyperthyroidism or malignancy.