The documentation says (doc):

Warning: On Windows, the returned value is a pseudo-handle for the current thread. It can't be used for numerical comparison. i.e., this function returns the DWORD (Windows-Thread ID) returned by the Win32 function getCurrentThreadId(), not the HANDLE (Windows-Thread HANDLE) returned by the Win32 function getCurrentThread().

However, GetCurrentThreadId (with a capital G, btw) function returns the thread identifier of the calling thread (MSDN) and GetCurrentThread (with a capital G, one more time) function returns a pseudo handle for the calling thread (MSDN).

QThread::currentThreadId really does return what GetCurrentThreadId returns:

Qt::HANDLE QThread::currentThreadId() Q_DECL_NOTHROW
{
    return reinterpret_cast<Qt::HANDLE>(quintptr(GetCurrentThreadId()));
}