...
uses
  Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,
  Dialogs, StdCtrls;

type
  TForm1 = class(TForm)
    Button1: TButton;
    Edit1: TEdit;
    procedure Button1Click(Sender: TObject);
  private
    { Private declarations }
  public
    { Public declarations }
  end;

var
  Form1: TForm1;

implementation

uses Unit2;

{$R *.dfm}

procedure TForm1.Button1Click(Sender: TObject);
var
  drawTH : testTH;
begin
  drawTH.Create(FALSE);
end;

...
uses
  unit1, Classes;

type
  testTH = class(TThread)
  private
    { Private declarations }
    procedure drawIt;
  protected
    procedure Execute; override;
  public

  end;

implementation

{ testTH }

procedure testTH.Execute;
begin
  Synchronize(drawIt);
end;

procedure testTH.drawIt;
begin
  Form1.Edit1.Text := 'geht';
end;