In galactorrhea-amenorrhea syndrome, the investigation you should advise apa from serum prolactin levels is:

Hyperprolactinemia leads to both Galactorrhea and Amenorrhea. Prolactin secretion is controlled by Prolactin inhibitor factor that is secreted from Hypothalamus, Vasoactive inhibitory peptide (VIP) and Thyroid releasing hormone (TRH) cause to increase Prolactin secretion. TRH in addition to increasing TSH causes rise in prolactin level. In patients with Primary hypothyroidism, increased levels of TRH can cause rise in prolactin levels.