The timing of the Korotkoff arterial sounds with reference to the electrocardiogram provides an objective index of thyroid function which is precise, accurate, rapid, innocuous, inexpensive, and independent of iodine metabolism. QKd is the interval between the QRS and the onset of Korotkoff sounds at diastolic pressure. The QKd value in 80 euthyroid subjects had a normal range of 186 to 235 msec. Ten patients with hypothyroidism had QKd values ranging from 230 to 320 msec.
Rodbard D, Fujita T, Rodbard S. Estimation of Thyroid Function by Timing the Arterial Sounds. JAMA. 1967;201(11):884-887. doi:10.1001/jama.1967.03130110110038