How long does it take for medical treatment to reduce the serum prolactin level? To reduce the size of the tumor?
The onset of action of dopamine agonists is rapid; because prolactin has a serum half-life of 50 minutes, a decrease in the prolactin level may be noted within 2 hours. However, normalization of prolactin levels may take weeks or months, with the maximal decrease usually seen by 3 months. Tumor size reduction can occur within 48 hours and may be demonstrated by improvement in visual fields when these are affected by the tumor. Tumor shrinkage is usually evident by 3 months; for larger tumors, it is recommended that MRI be performed again at this time, to ensure that there is an adequate response to the treatment. Maximal tumor shrinkage, however, is not usually observed until after at least 12 months of treatment. Repeat MRI after 1 year of treatment is, therefore, recommended.