FirstEnergy (FE) shares slumped 9.5% in recent Thursday trading, a day after the company said it will adopt core earnings as the basis for measuring growth performance to exclude volatile items starting this year.

To be excluded from the profit measurement are the unregulated investment in the Signal Peak coal mine and pension income, the company said Wednesday in a statement with Q4 earnings.

The shift "was a very thoughtful decision and is the result of the volatility in these two income streams that, quite frankly, we don't control," Chief Financial Officer K. Jon Taylor said Thursday on the earnings call, according to a FactSet transcript.

In Q4, he said Signal Peak's profitability was adversely affected by the decline in commodity prices, while high interest rates negatively affected the pension plan.

"The contribution of these two items have declined significantly from our expectation, and this type of volatility is not sustainable and the reason for the change to reporting on core earnings," Taylor said.

The utility company's Q4 non-GAAP earnings and revenue missed estimates by analysts.

Price: 38.96, Change: -4.09, Percent Change: -9.50