Thyroid Gland: Location, Hormones, Functions, and Common Disorders

The thyroid gland is a butterfly-shaped endocrine organ located in the anterior (front) neck, just below the laryngeal prominence ("Adam's apple") and anterior to the trachea. It consists of two lobes connected by a narrow isthmus.

Hormones Produced

• Thyroxine (T4): Primary secretory product (~80%); prohormone converted to T3 in tissues
• Triiodothyronine (T3): Biologically active form; ~20% secreted directly, rest from peripheral T4 conversion
• Calcitonin: Secreted by parafollicular (C) cells; lowers blood calcium (minor role in humans)

Physiological Roles

• Metabolic regulation: T3/T4 increase basal metabolic rate of lipids, carbohydrates, proteins
• Growth and development: Critical for brain development in fetal/neonatal life; bone growth
• Cardiovascular: Increase heart rate, contractility, cardiac output
• Thermogenesis: Regulate body temperature
• GI motility, skin/hair turnover, reproductive function

Regulation

Hypothalamus (TRH) → Anterior pituitary (TSH) → Thyroid (T4/T3) → Negative feedback on TRH/TSH. Iodine is essential for hormone synthesis.

Common Disorders

• Hypothyroidism: Low T4/T3, high TSH (Hashimoto's, iodine deficiency)
• Hyperthyroidism: High T4/T3, low TSH (Graves', toxic nodules)
• Goiter: Thyroid enlargement (iodine deficiency, nodules, autoimmune)
• Thyroid nodules/cancer: Usually asymptomatic; evaluated with ultrasound ± FNA