type
  TOnSomethingHappenedEvent = procedure(var OldValue: string; const NewValue: string) of object;
  
  TMyThread = class(TThread)
  private
    FText: string;
    FOnSomethingHappened: TOnSomethingHappenedEvent;
    procedure DoOnSomethingHappened;
    ...
  public
    ...
    property OnSomethingHappened: TOnSomethingHappenedEvent read FOnSomethingHappened write FOnSomethingHappened;
  end;
  
procedure TMyThread.DoOnSomethingHappened;
var
  OldValue: string;
begin
  if Assigned(FOnSomethingHappened) then
    begin
      FOnSomethingHappened(OldValue, FText);
      Log('Changed from ' + OldValue + ' to ' + FText);
    end;
end;

procedure TMyThread.Execute;
begin
  ...
  Synchronize(DoOnSomethingHappened);
end;