  We study the space of periodic solutions of the elliptic $\sinh$-Gordon
equation by means of spectral data consisting of a Riemann surface $Y$ and a
divisor $D$. We show that the space $M_g^{\mathbf{p}}$ of real periodic finite
type solutions with fixed period $\mathbf{p}$ can be considered as a completely
integrable system $(M_g^{\mathbf{p}},\Omega,H_2)$ with a symplectic form
$\Omega$ and a series of commuting Hamiltonians $(H_n)_{n \in \mathbb{N}}$. In
particular we relate the gradients of these Hamiltonians to the Jacobi fields
$(\omega_n)_{n\in \mathbb{N}_0}$ from the Pinkall-Sterling iteration. Moreover,
a connection between the symplectic form $\Omega$ and Serre duality is
established.
