The datasheet only specifies this at 20KHZ.
Here the answer from the Productline - the values are CALCULATED.
The THD at 20 KHz and Vout = 20 Vp-p is 0.002%, which is good. It will be
better at 1 KHz with Vout = 1.7V. I can calculate the THD for 20 KHz with Vout
= 20 Vp-p using just HD2 and HD3 from the plots in TPC12 of the AD8113 data
sheet, and it comes out to be about 0.002%. The higher harmonics really don’t
contribute much (the number of harmonics is not specified, though 5 is
common). I can similarly calculate the THD at 1 KHz with Vout = 20 Vp-p using
the same plot. This comes out to be about 0.0015%. It will be considerably
better with Vout = 1.7V – probably well below 0.001%.