Fast Fouriet Transformation and Total Harmonic Distortion

I would like to ask for help regarding the fast Fouriet Transformation and the Total Harmonic Distortion of a signal I have saved in the Matlab workspace from a simulink running. I have a current signal "Ira" and the corresponding "time" array. I have extracted a single period data from the vectors and I used the command :

"THD_db = thd(Ira);"

Of course it gave me a meaningless result.

I have tried to analyze the spectrum of the signal by using the following commands: "Ira_fft = abs(fft(Ira_fft));"

There is something I did not understand about how to use this function for the purpuose I want to achieve.

Could anyone tell me how to use them properly?

Ps. I could not use PowerGui for the signal analysis because I have the basic version of Matlab which does not include PowerGui block.

Thanks in advance