Thyroid Gland — Location, Parts and Dimensions
Introduction
The thyroid gland is the largest endocrine gland in the body. It secretes three hormones:
- Triiodothyronine (T3) and Tetraiodothyronine / Thyroxine (T4) — regulate basal metabolic rate; essential for normal growth and development
- Calcitonin (Thyrocalcitonin) — reduces blood calcium by decreasing renal tubular reabsorption; opposite to parathormone
Special features:
- Only endocrine gland situated superficially — accessible for physical examination
- Only endocrine gland dependent on external environment (dietary iodine) for hormone synthesis
- Only endocrine gland that stores its hormones in follicles and releases them as needed
- One of the most vascular organs in the body
Location
The thyroid gland lies in the lower part of the front and sides of the neck, opposite to vertebrae C5, C6, C7, and T1, clasping the upper part of the trachea.
It is an H-shaped gland consisting of:
- Right and left lateral lobes (vertical)
- A horizontal isthmus connecting them across the midline
Parts
Lateral Lobes (2)
Each lobe is conical (pyramidal) in shape and presents:
- One apex — directed upwards and slightly laterally; limited superiorly by attachment of sternothyroid to the oblique line of thyroid cartilage
- One base — at the level of the 5th or 6th tracheal ring
- Three surfaces: lateral (superficial), medial, and posterolateral
- Two borders: anterior (thin) and posterior (thick and rounded)
Extent: From the oblique line of the thyroid cartilage above to the 5th or 6th tracheal ring below.
Isthmus
- Connects the lower parts of the two lateral lobes
- Lies in front of the 2nd, 3rd, and 4th tracheal rings
- Has two surfaces (anterior, posterior) and two borders (superior, inferior)
Pyramidal Lobe
- Present in ~50% of individuals
- Projects upwards from the isthmus — usually to the left of midline
- Represents the lower end of the persistent thyroglossal duct
- May be connected to the hyoid bone by a fibrous/fibromuscular band — levator glandulae thyroideae
Dimensions and Weight
| Structure | Dimensions |
|---|---|
| Each lateral lobe | 5 cm long × 3 cm wide × 2 cm thick (Vishram) |
| Isthmus | 1.25 cm × 1.25 cm (both vertical and transverse) |
| Weight of gland | ~25 g |
The gland is larger in females, and further increases in size during menstruation and pregnancy.
Why Thyroid Swellings Move on Swallowing
The false capsule of the thyroid gland is derived from the pretracheal fascia, which is attached to the thyroid and cricoid cartilages and the hyoid bone above. These structures move upward during deglutition, carrying the thyroid gland with them.
This is the basis for clinically distinguishing thyroid swellings from other neck lumps — thyroid swellings move upward on swallowing.