I want to know what makes the balance of the Prolactin abnormal. Is that related to the presence of a nodule near the pituitary?
The main abnormality in prolactin levels is hyperprolactinemia, meaning blood levels of prolactin above the normal range, not during pregnancy or lactation.
The major cause of these abnormal prolactin levels are tumors consisting of pituitary lactotroph cells--called prolactinomas--which secrete prolactin. This is generally corrected with synthetic dopamine analogues, as dopamine negatively regulates secretion of prolactin in lactotroph cells.
Here is a 2010 review with further detail: http://joe.endocrinology-journals.org/content/206/1/1.full.pdf
I am not sure whether I understand your question right, but prolactin levels can be abnormal under several conditions like cancer, drug abuse, stress, breast surgery etc.
Its secretion is mainly controlled (inhibited) by Dopamine. Other hypothalamic factors like VIP, TRH, PRH etc. are enhancers.
Please specify your question for a detailed answer.
Prolactin is a hormone which is produced by the pituitary gland. Prolactin is named because it was found to be elevated in women who are lactating, its also produced by men and in elevated levels during sleep. Because of this, like many blood factors and hormones it probably has more than a single purpose.
When there is an abnormal growth from the pituitary gland, since the tissue produces prolactin, abnormal pituitary tissue can also produce extraordinary amounts of prolactin. I don't think this means that its a cancer, but its a sign that there is an unusual pituitary condition. It can even be the result of taking some drugs.
Most of these tests are from experience - we find that this happens a lot, but the causes are usually not understood well. Cancers in particular are quite unpredictable and can have many different properties from one case to another. Pituitary tumors are the most common central nervous system tumors, so common they are found in autopsies 3-27% of the time, often never having been found previously.