There is a common suite of symptoms that bring men over to Peak Testosterone or to the Peak Testosterone Forum, such as fatigue, low libido, erectile dysfunction, mental fog, loss of morning erections and depression - been there myself! One thing that is problematic is that there are many conditions and underlying root issues that can produce these symptoms. And trying to discover the cause of these kind of more vague symptoms is not something doctors usually want or, frankly, are often equipped to handle. And a perfect example is prolactin.
Prolactin is something that should be pulled much, much more frequently by physicians in my opinion. It's a cheap test and high and low values are surprisingly common. And, as I will show below, high and low values can show the underlying cause of many common chronic conditions. However, with health care costs spiraling out of control, most docs rarely pull it. For example, I was low testosterone for probably five years before anyone pulled my prolactin. This is really troubling, because I had almost every symptoms above.
For those of you who may be struggling unnecessarily, I have made pulling prolactin part of my "Peak Testosterone Program," and below is STEP 6: Six Great Reasons to Monitor and Test Prolactin in Men:
1. High Prolactin Can Lower Testosterone and Libido. One study used testosterone less than 400 ng/dl and/or low libido as a sign of a man with possible high prolactin.  Of course, the reason is that, as prolactin rises, it generally tends to lower testosterone. Dopamine and prolactin have a yin and yang relationship. Thus, as prolactin rises, dopamine will probably be lower and, of course, that is never good for one's sex life (or career). Low dopamine is associated with many psychological conditions, including some forms of depression, OCD, ADHD, neuroticism, etc. (Low dopamine and RLS (Restless Leg Syndrome also often occur together.)
Many young men, who should have a raging libido, show up at the doctor's office not knowing why they have little to no desire for sex. Most doctors will hand them a Cialis or recommend counseling. Why not pull prolactin instead of just making huge assumptions?
NOTE: Another common problem that I see on the Peak Testosterone Forum is men in the upper part of the lab range for prolactin. Of course, their prolactin is not high enough to warrant one of the powerful medications such as cabergonine, since it is often full of side effects. Yet my experience is that even moderately high levels like this can cause some men issues. Again, isn't it better to know the root cause?
CAUTION: Macuna pruriens, according to one study anyway, can lower prolactin and raise testosterone by about 30% and will likely provide some relief to these men. You can read about it on my page on How to Lower Prolactin Naturally. As a caution, though, a couple of men on our forum got arrhythmias (heart palpiations) from taking macuna, so please discuss with your doctor first.
2. High Prolactin May Be a Sign of Zinc Deficiency. Some men may have high prolactin, because they just need some zinc. Again, pulling prolactin might be a way to discover this hidden issue. Zinc is an expensive supplement and low levels can negiatively impact literally dozens of metabolic pathways in the body.
3. High Prolactin Can Cause Gynecomastic, a.k.a. "Man Boobs." The same study mentioned above also used gynecomastia as a possible sign of high prolactin levels.  Many men are struggling with gyno and so why not pull this number if that is the case? The good news is that, even if the doctor does not pull a man's prolactin, it is fairly common to prescribe tamoxifen for gyno and one study showed that this lowered prolactin. See my page on Tamoxifen and Prolactin for additional information.
4. High Prolactin Can Be a Sign of a Prolactinoma. Prolactinomas seem to be a fairly common kind of tumor. They are usually benign but can cause visual issues and headaches if they get large enough. Again, we have had quite a few posters who have had some kind of pituitary tumor.
5. Low Prolactin May Signal Metabolic Syndrome: It might seem low prolactin would be a good thing - a sign of high dopamine and high testosterone. Unfortunately, that is usually not the case. In fact, low prolactin is actually strongly associated with Metabolic Syndrome according to one recent Journal of Sexual Medicine study.  It is also linked with arterial plaque! So, in actuality, low prolactin is not only something non-desireable, but is usually dangerous. If you do have low prolactin, then you may want to look for other signs of fatty liver, prediabetes and insulin resistance here: Blood Glucose and Insulin Pages.
6. Low Prolactin May Cause Premature Ejaculation. Low dopamine and high prolactin can cause anorgasmia in some men or delayed orgasm in still others. See my page on Men That Cannot Orgasm for more information.
7. High Prolactin Can Be Caused By Hypothyroidism. Check your thyroid hormones as low thyroid function can mildly elevate prolactin levels. Obviously, there can be synergies in symptoms, since high prolactin and hypothyroidism can lower testosterone, dopamine and libido and also increase fatigue. One of our forum members wrote the following story his personal experience with exactly this issue:
"I got 4 tests with prolactine at 32-48 and tsh 1.2-4, then i have 4 tests with prolactine in the middle of range with tsh 0.07-0.53, i also noticed this winter i got swollen breasts when i didnt use levothyroxine and the breasts returned to normal in a few days when i started with levothyroxine. Not sure if my memory works but i think range for prolactin is 4-20 g/L" 
1) The Journal of Urology, 1997, 158(5):1764-1767, "Endocrine screening in 1,022 men with erectile dysfunction: clinical significance and cost-effective strategy."
2) 4) J Sex Med, 2009 May, 6(5):1457-66, "Hypoprolactinemia: a new clinical syndrome in patients with sexual dysfunction"