Problem with user-defined non-italic theorem in the article class

[Paul Smith]

Dear All

I am using the following code in the preamble to get non-italic
theorems in the article class:

\newtheorem{prethm}{Theorem}
\newenvironment{thm}
  {\begin{prethm}\upshape}{\end{prethm}}

However, 

\begin{thm}[theauthor]
blalalalalal
\end{thm}

does not produce what one expects with the normal theorem environment;
I get the following:

Theorem 1 [theauthor] 
  blalalalala

[theauthor] is not written in bold as it would be supposed.

Any ideas?

Thanks in advance,

Paul