17.4 The Thyroid Gland

Thyroid Gland Anatomy and Function

The thyroid gland is a butterfly-shaped organ in the anterior neck that produces hormones controlling metabolism, growth, and development. Because thyroid hormones affect nearly every tissue in the body, understanding this gland's structure, hormone synthesis, and regulation is essential for connecting endocrine function to whole-body physiology.

Structure of the Thyroid Gland

The thyroid sits anterior to the trachea and inferior to the larynx, roughly at the level of the C5–T1 vertebrae. It consists of two lateral lobes (each about 5 cm long and 2.5 cm wide) connected by a thin bridge of tissue called the isthmus. The whole gland weighs about 20–30 g.

Blood supply and innervation:

• Arterial supply comes from the superior thyroid arteries (branches of the external carotid) and the inferior thyroid arteries (from the thyrocervical trunk)
• Venous drainage flows through the superior, middle, and inferior thyroid veins
• Autonomic innervation includes both sympathetic and parasympathetic fibers, which primarily regulate blood flow to the gland rather than hormone secretion directly

Histology is where the real functional story begins. The thyroid is made up of spherical follicles, each lined by a single layer of follicular cells (thyrocytes). These follicles are filled with a protein-rich substance called colloid, which stores the precursor material for thyroid hormones. Scattered between the follicles are parafollicular cells (C cells), which produce calcitonin.

Production of T3 and T4 Hormones

Thyroid hormone synthesis is a multi-step process that depends on dietary iodine. Here's how it works:

1. Iodide trapping: Follicular cells actively transport iodide ($I^-$) from the blood into the cell using the sodium-iodide symporter (NIS) on their basolateral membrane.
2. Oxidation and organification: The enzyme thyroid peroxidase (TPO) oxidizes iodide and attaches it to tyrosine residues on a large protein called thyroglobulin (Tg), which sits in the colloid. Adding one iodine creates monoiodotyrosine (MIT); adding two creates diiodotyrosine (DIT).
3. Coupling: Still within the thyroglobulin molecule, MIT and DIT residues combine. DIT + DIT produces T4 (thyroxine), while MIT + DIT produces T3 (triiodothyronine).
4. Storage: T3 and T4 remain bound to thyroglobulin in the colloid until the gland is stimulated to release them.
5. Secretion: When TSH from the anterior pituitary signals the follicular cells, they endocytose colloid containing thyroglobulin. Lysosomes inside the cell break down the thyroglobulin, freeing T3 and T4, which are then released into the bloodstream.
6. Transport: In the blood, most T3 and T4 travel bound to thyroid-binding globulin (TBG). Only the small free (unbound) fraction is biologically active.

The thyroid produces much more T4 than T3, but T4 is relatively inactive. Deiodinase enzymes in peripheral tissues (liver, kidneys, muscle) convert T4 to the more potent T3, which is the form that acts on target cells.

Regulation: The Hypothalamic-Pituitary-Thyroid (HPT) Axis

Thyroid hormone levels are tightly controlled through a three-tier feedback loop:

1. The hypothalamus secretes thyrotropin-releasing hormone (TRH).
2. TRH travels to the anterior pituitary and stimulates release of thyroid-stimulating hormone (TSH).
3. TSH acts on the thyroid gland, promoting iodide uptake, hormone synthesis, and secretion of T3 and T4.

Negative feedback keeps the system in balance: when circulating T3 and T4 levels rise, they inhibit both TRH release from the hypothalamus and TSH release from the anterior pituitary. This reduces thyroid stimulation and brings hormone levels back down. Conversely, low T3/T4 levels remove that inhibition, allowing TRH and TSH to rise and drive more hormone production.

Thyroid Hormones in Metabolism Regulation

Thyroid hormones act on virtually every organ system. Their effects are broad because they regulate gene expression related to energy use and oxygen consumption.

• Basal metabolic rate (BMR): T3 and T4 increase BMR by stimulating carbohydrate, protein, and lipid metabolism. They boost oxygen consumption and heat production, which is why hypothyroid patients often feel cold and hyperthyroid patients feel overheated.
• Cardiovascular system: Thyroid hormones increase heart rate, cardiac output, and blood flow through vasodilation. They also increase the heart's sensitivity to catecholamines (epinephrine and norepinephrine), which is why hyperthyroidism can cause palpitations and tachycardia.
• Growth and development: These hormones are essential for normal skeletal growth and, critically, for brain development during fetal and neonatal life. Congenital hypothyroidism left untreated can cause irreversible intellectual disability (historically called cretinism).
• Other effects:
  • Increased intestinal motility and nutrient absorption
  • Enhanced speed of muscle contraction and relaxation
  • Stimulation of bone remodeling

Calcitonin and Parafollicular Cells

Parafollicular cells (C cells) are neuroendocrine cells derived from the neural crest during embryonic development. They sit between the thyroid follicles and produce the hormone calcitonin.

Calcitonin is released when calcium-sensing receptors on C cells detect elevated blood calcium levels. It works to lower blood calcium by:

• Inhibiting osteoclast activity, which slows bone resorption (the breakdown of bone that releases calcium into the blood)
• Promoting renal calcium excretion, so more calcium is lost in urine
• Opposing parathyroid hormone (PTH), which raises blood calcium

That said, calcitonin's role in day-to-day calcium homeostasis in adults is relatively minor compared to PTH and vitamin D. Its clinical significance is mainly as a tumor marker: elevated calcitonin levels can indicate medullary thyroid cancer (MTC), a malignancy that arises specifically from parafollicular cells.

Thyroid Disorders

• Goiter: Enlargement of the thyroid gland. Common causes include iodine deficiency (the gland enlarges trying to trap more iodide) and autoimmune conditions such as Graves' disease or Hashimoto's thyroiditis.
• Hypothyroidism: An underactive thyroid producing insufficient T3/T4. Systemic effects include fatigue, weight gain, cold intolerance, bradycardia, and constipation. Hashimoto's thyroiditis is the most common cause in iodine-sufficient regions.
• Thyroid storm: A life-threatening emergency involving severe thyrotoxicosis, typically triggered in patients with untreated or poorly managed hyperthyroidism. Signs include dangerously high fever, tachycardia, agitation, and potential cardiovascular collapse. This requires immediate medical intervention.