Switching a current source on and off
Valid for a unipolar 0 to 3.3V signal with a 1.7V drop at low current, you need to use a PFET on V+ to disable it.

Vt (to Vgs.th)ought to be &lt;2V.

Another method is a PFET or  PNP or Quad bilateral CMOS switch on the Iout with suitable 0V or 5V enable signal.

But if your source impedance is this high, you will want to consider CMRR. Then use an INA and a high L balun to eliminate CM noise with STP or active guarding or passive guarding like EEG/ECG signals.