What is the normal size of a thyroid gland?

Normal Thyroid Gland Size

The normal thyroid gland volume is approximately 13 mL overall (12.09±2.05 mL in females and 14.53±2.55 mL in males), with an upper limit of 18 mL for women and 25 mL for men. 1, 2

Anatomical Dimensions

• The average length of the thyroid lobes is approximately 4.32 cm for the right lobe and 4.22 cm for the left lobe 3
• The average thickness of the lobes is 1.13 cm for the right lobe and 1.18 cm for the left lobe 3
• The transverse diameter of the thyroid gland averages 28.79 mm (SD 6.57 mm), with the right lobe averaging 15.05 mm and the left lobe averaging 13.74 mm 4
• The right thyroid lobe is typically larger than the left lobe in both males and females 1

Variations in Thyroid Morphology

• The thyroid gland can present in various shapes: lateral lobes with isthmus (50.2%), horseshoe-shaped (36.8%), separate lobes without isthmus (7.9%), and irregularly shaped (5.1%) 5
• A pyramidal lobe is present in approximately 37.77% of individuals, most frequently arising from the left lobe 3
• The isthmus may be absent in about 16.66% of cases 3
• The isthmus location can vary from the cricoid cartilage to the fourth tracheal ring 3

Factors Affecting Thyroid Size

• Thyroid volume correlates significantly with height, weight, and body surface area in both sexes 1
• Geographic and regional variations exist in thyroid size, with studies showing smaller gland sizes in some populations compared to Western Caucasian and Japanese subjects 5
• Iodine sufficiency in a region can influence thyroid size, with iodine-deficient areas typically showing larger thyroid volumes 5

Clinical Significance of Thyroid Size

• Thyroid nodules smaller than 1 cm are generally classified as "small" and often have different management approaches compared to larger nodules 6
• Nodules less than 1 cm with no suspicious features are typically recommended for surveillance rather than immediate biopsy 6
• Fine needle aspiration cytology (FNAC) is recommended for thyroid nodules larger than 1 cm, regardless of thyroid function test results 7
• The size of thyroid nodules alone is not a reliable indicator of malignancy, though larger nodules have a higher risk potential 6

Important Considerations in Thyroid Evaluation

• Ultrasound is the preferred imaging modality for accurate assessment of thyroid size and nodule characteristics 7
• When evaluating thyroid size, it's important to consider the patient's body size, sex, and geographic region, as these factors can influence what is considered "normal" 1, 5
• Knowledge of normal thyroid dimensions is crucial for surgical planning, evaluation of thyroid disorders, and interpretation of imaging studies 3

Pitfalls in Thyroid Size Assessment

• Using universal standards for thyroid volume without considering individual factors like body size, ethnicity, and regional iodine status may lead to misclassification 2, 5
• Overdiagnosis of thyroid abnormalities based solely on size can lead to unnecessary interventions and psychological burden for patients 8
• The relationship between thyroid size and function is not always direct, as even normal-sized thyroids can have functional abnormalities 8