23rd May 2025

The Thyroid Gland

Anatomy

The thyroid gland consists of a midline isthmus horizontally (below the cricoid cartilage) and two lateral lobes extending upwards over the lower half of the thyroid cartilage . The gland lies deep to the strap muscles, enclosed in the pretracheal fascia, which anchors it to the trachea and means the thyroid moves upwards on swallowing. 

Anatomical relations

• Anteriorly: infrahyoid muscles
• Laterally: carotid sheath, comprising of the common carotid artery, internal jugular vein, and vagus nerve

Medially: larynx, pharynx, trachea, and oesophagus, and the external and recurrent laryngeal nerves (the left and right recurrent laryngeal nerves are in close proximity to the thyroid gland, and may be damaged by compression (e.g. by a nearby mass) or during thyroidectomy)

Physiology

Iodine is a substrate for thyroid hormone biosynthesis. It is actively transported into the follicular layer and produces mono-iodotyrosine (MIT) and diiodotyrosine (DIT). Thyroid peroxidase links two DITs to form T4, and a DIT and MIT to form T3 and reverse T3. The thyroid’s main product is T4 (inactive form) and it also secretes 20% of circulating T3 (active form); the remainder is generated in extra-glandular tissues by the conversion of T4 to T3 (mainly in the liver and kidneys). In the blood, T3 and T4 are bound to the plasma protein. Production of  the thyroxine hormones and its function can be affected by the following:

• Thioureas (e.g., carbimazole and propylthiouracil) and mercaptoimidazole inhibit the initial oxidation of iodide and coupling of iodothyronines.

• In large doses, iodine itself blocks organic binding and coupling reactions.

• Lithium has several inhibitory effects on intrathyroidal iodine metabolism.

Thyroid hormones function

In general, they increase metabolic rate where oxygen, energy consumption and thermogenesis increase. Effects on target organs are as follows:

• Cardiac: 
  • Increase the expression and activity of cardiogenic B receptors
  • Results in increased heart rate, stroke volume, cardiac output, and contractility.
• Respiratory: 
  • Increase respiratory centre stimulation and lung perfusion
  • Increases respiratory rate and oxygenation. 
  • Thyroxine also stimulates surfactant production in new-borns.     
• Neurological: 
  • Stimulate the maturation of the nervous system.
• Musculoskeletal: 
  • Stimulate bone growth and the development of type II muscle fibres.
• Endocrine: 
  • Promote fertility, ovulation, and menstruation.
• Metabolism:      
  • increase the basal metabolic rate (BMR) leading to an increase in oxygen consumption and body temperature, with an additional thermoregulation function. 
  • stimulate protein anabolism. 
  • increase glucose reabsorption, gluconeogenesis, glycogenolysis, and glucose oxidation, leading to increased carbohydrate metabolism
  • stimulate lipolysis or lipogenesis depending on metabolic status.