[Tex/LaTex] Itemizing theorem body

Question

Using package amsthm, the desired theorem style achieved,

\documentclass{book}
\usepackage{amsthm}

\newtheoremstyle{mystyle}{}{}{}{}{\bf}{}{\newline}{}
\theoremstyle{mystyle}
\newtheorem*{mythm}{Theorem}

\begin{document}
\begin{mythm}
This is a normal body text.
\end{mythm}
\end{document}

But itemizing the body makes trouble,

\documentclass{book}
\usepackage{amsthm}

\newtheoremstyle{mystyle}{}{}{}{}{\bf}{}{\newline}{}
\theoremstyle{mystyle}
\newtheorem*{mythm}{Theorem}

\begin{document}
\begin{mythm}
    \begin{itemize}
        \item This an itemized body text.
        \item This an itemized body text.
    \end{itemize}
\end{mythm}
\end{document}

As the last output shows, first item has jumped to the head line.

How can I bring it back to the body?

Answer 1

Here is a solution with enumitem. One can define a thmitemise clone, which incorporates by default the given setup, to avoid having to type it each time it is used:

\documentclass{book}
\usepackage{amsthm} \newtheoremstyle{mystyle}{}{}{}{}{\bf}{}{\newline}{}
\theoremstyle{mystyle}
\newtheorem*{mythm}{Theorem}
\usepackage{enumitem, showframe}

\begin{document}

\begin{mythm}
    \begin{itemize}[wide=0.5em, leftmargin =*, nosep, before = \leavevmode\vspace{-\baselineskip}]
        \item This an itemized body text.
        \item This an itemized body text.
    \end{itemize}
\end{mythm}
\end{document}