Response to Toraih et al. re: “Chasing π-fection in Thyroidology”

Dear Editor:

We are responding to Dr. Toraih and colleagues' Letter to the Editor entitled Chasing π-fection in Thyroidology ¹ in our capacity as members of the Clinical Affairs Committee and the Surgical Affairs Committee of the American Thyroid Association (ATA).

The authors raise a valid point that the ATA thyroid nodule calculator may underestimate the volume calculation of thyroid nodules due to the variability in nodule shape. Indeed, the commonly accepted method to estimate the volume of a thyroid nodule is to multiply its tridimensional linear measurements by π/6, a formula that has been used in many published studies. ^2,3 Interestingly, the same formula has been applied to volumetry of the thyroid gland, although the appropriate value for the correction factor has been debated. ⁴ While this would tend to support the request to update the calculator on the ATA website, we wish to highlight several important caveats.

First, consistency is paramount when serial measurements are used to assess nodule growth, so prior volumes would have to be recalculated using the new method. Alternatively, the current formula could continue to be employed for nodules already under surveillance. Therefore, we favor the proposal to add a calculator rather than replace the existing one and ensure they are clearly labeled.

Interobserver variability in volume measurements also must be considered. Some investigators have found variation as high as 49%, ⁵ but a more recent study of 85 well-defined nodules showed a variability of 13.1%. ² However, since volume estimates are typically based on linear dimensions, accuracy depends on the clarity of the interface between the nodule and surrounding parenchyma.

In addition, while the ellipsoid formula may be commonly employed for volume calculation, thyroid nodules are often not ellipsoidal in shape, so the calculation may not be accurate for a large fraction of thyroid nodules. ⁶

Practitioners may perform the volume calculation themselves or use the calculated volume provided by the ultrasound machine. Most modern ultrasound machines can automatically calculate the volume of thyroid nodules after three measurements have been made. Practitioners should consider asking manufacturers to verify the formula used by their equipment to assure uniformity, particularly in settings where more than one vendor's scanner is available. We also note that while three-dimensional sonography has shown promise in measuring the volume of thyroid nodules, ⁷ and artificial intelligence may ultimately improve precision, ⁸ these techniques are not yet widely deployed.