tyreoidea

Tyreoidea, or the thyroid gland, is a butterfly-shaped endocrine organ in the neck. It consists of two lobes connected by a narrow isthmus and lies anterior to the trachea, inferior to the larynx. It is highly vascular and responsive to hormonal regulation.

Histologically the gland is composed mainly of spherical follicles lined by cuboidal to columnar follicular cells, surrounding colloid that stores thyroglobulin. Interspersed parafollicular (C) cells secrete calcitonin. Follicular cells synthesize thyroid hormones by iodinating thyroglobulin and releasing triiodothyronine (T3) and thyroxine (T4).

Thyroid hormones regulate basal metabolic rate, protein synthesis, and growth and development. They are crucial for normal brain development and myelination during fetal and early postnatal life. Hormone production depends on iodine intake and is regulated by the hypothalamic–pituitary–thyroid axis, with TSH stimulating synthesis and release.

This axis responds to thyrotropin-releasing hormone (TRH) from the hypothalamus and yields variable serum TSH and free T4/T3 levels. Abnormal thyroid function can result from autoimmune disease, iodine deficiency or excess, drugs, or nodular disease. Calcitonin's role in humans is less central but may influence calcium homeostasis.

Common disorders include hypothyroidism (often autoimmune or iatrogenic), hyperthyroidism (notably Graves’ disease), and thyroid nodules or goiter. Thyroid cancer types include papillary, follicular, medullary, and anaplastic. Evaluation uses TSH and free T4, imaging (ultrasound, scintigraphy), and sometimes fine-needle biopsy. Treatments include levothyroxine for hypothyroidism, antithyroid drugs or radioactive iodine for hyperthyroidism, and surgery for cancer or large goiters.