The prevailing notion of the normal thyroid size holds that the thyroid should not be discernible over the neck surface nor be felt upon palpation by the doctor’s hands during examination. However, findings from ultrasonographic volumetry have revealed that a great variety of thyroid sizes exist although they may not be palpable.
It appears that the normal thyroid size varies in different parts of the world. In countries where there is iodine deficiency, the normal thyroid size is larger. In countries with abundant iodine intake such as Japan, Iceland, Sweden, and the United States, the thyroid gland appears to be of “minimum size.” Thus, the thyroid is often invisible in most people in the West.
Ultrasonography defines normal thyroid size with more precision and reliability. Ultrasonography is a safe technology. It is portable and easy to use, thus it is replacing palpation in evaluating the thyroid. Through the help of ultrasonography, some leading authorities define the normal thyroid as one whose growth mechanisms have not required extra stimulation to overcome adverse influences.
In addition to iodine intake, other factors that affect the normal thyroid size include serum TSH concentration, body mass index and parity. Your age, gender and whether you are a cigarette smoker also contribute to the size of your thyroid. Other factors are still unknown.
Variation in thyroid volume with respect to age depends on iodine availability. Where iodine supply tends to be deficient, thyroid volume may increase until age 40-45 and generally remains stable afterwards. In areas with iodine sufficiency, the gland tends to diminish in volume after age 40.
Normal thyroid size may also be different for males and females, with bigger volumes usual among men. This pattern may reflect the correlation between thyroid volume and body mass (or the body mass index). There is a different gender-specific influence on female thyroid volumes during pregnancy.
There have been scientific studies linking smoking and a bigger thyroid volume. There is a stronger association between the two in iodine-deficient areas, although this is not yet a settled issue.
Individual variation in normal thyroid size is also heavily influenced by genetics. Your genetic make-up influences the number, receptivity, and efficiency of the TSH (thyroid stimulating hormone) receptors that affect the secretion of the cells and also the proliferation and growth of thyroid gland cells.
In a sense then, when the doctor says your thyroid is normal, this assessment is relative to the thyroid volumes in your part of the world and also on individual factors like your age, gender, and genetics. There cannot be a one-size-fits-all normal thyroid size.
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