type
  TMyThread = class(TThread)
    FHash: ^string;
    FName: string;
    constructor Create(CreateSuspended: Boolean; Filename:string; var Hash: string);
    procedure Execute; override;
    procedure Update;
end;

type
  TMyFile = class(TObject)
  public
    OriginalName: string;
    Hash: string;
    constructor Create;
    destructor Destroy; override;
    function CalculateHash:string;
end;

implementation

constructor TMyThread.Create(CreateSuspended: Boolean; Filename:string; var Hash: string);
begin
  FName:=Filename;
  FHash:=@Hash;
  inherited Create(CreateSuspended);
end;

procedure TMyThread.Execute;
begin
 while not Terminated do
 begin
    FHash^:=MD5DigestToStr(MD5File(FName));
    Terminate;
 end;
 Synchronize(Update);
end;

procedure TMyThread.Update;
begin
  //Showmessage(FHash^);  // РАссчитанный хэш выводится нормально
end;
//===================================================
function TMyFile.CalculateHash:string;
var DT: TMyThread;
begin
  //if FileExists(OriginalName) then result:=MD5DigestToStr(MD5File(OriginalName)) else result:='';   //Выполнение этой операции выношу в отдельный поток
  DT := TMyThread.Create(true,OriginalName,Hash);
  DT.FreeOnTerminate:= True;
  DT.Execute;
end;
//=================[Из другого юнита]==================================
procedure TForm1.N1Click(Sender: TObject);
var DF: TMyFile;
begin
  if OpenFile.Execute then
  begin
    DF:=TMyFile.Create;
    DF.OriginalName:=ExtractFilename(OpenFile.FileName);
    DF.Hash:=DF.CalculateHash;
    while df.Hash='' do begin end;     /////Этот цикл не прекращается
    showmessage(df.Hash);
    DF.Destroy;
  end;
end;

//===================================================================
// ЮНИТ формы. Здесь просто беру файл и путь передаю потоку.
type
  TMyThread = class(TThread)
    FName: string;
    //FHash: string;     //для отладки
    constructor Create(CreateSuspended: Boolean; Filename:string);
    procedure Execute; override;
    procedure Update;
end;

procedure TForm1.N1Click(Sender: TObject);
var DT: TMyThread;
begin
  if OpenFile.Execute then
  begin
    DT:=TMyThread.Create(true,OpenFile.FileName);
    DT.FreeOnTerminate:=true;
    DT.Execute;
  end;
end;
//==============================================================================
constructor TMyThread.Create(CreateSuspended: Boolean; Filename:string);
begin
  FName:=Filename;
  inherited Create(CreateSuspended);
end;

procedure TMyThread.Execute;
var DF: TMyFile;
begin
  DF:= TMyFile.Create;
  while not Terminated do
  begin
    DF.OriginalName:=FName;
    DF.Hash:=DF.CalculateHash;
    //FHash:=DF.Hash; // для отладки выдёргиваю значение для Update
    Synchronize(Update);
    Terminate;
  end;
  DF.Destroy;
end;

procedure TMyThread.Update;
begin
  //Showmessage(FHash); для отладки проверяю - Значение приходит
end;
//=================[Из другого юнита]==================================
type
  TMyFile = class(TObject)
  public
    OriginalName: string;
    Hash: string;
    constructor Create;
    destructor Destroy; override;
    function CalculateHash:string;
end;

function TMyFile.CalculateHash:string;
begin
  if FileExists(OriginalName) then result:=MD5DigestToStr(MD5File(OriginalName)) else result:=''; 
end;