We had an 18-year-old man on our Peak Testosterone Forum who reported huge swings in prolactin:
"Prolactin was: 6.5 ng/ml, 33ng/ml, 16ng/ml, 7ng/ml, 7.1 ng/ml (2-17ng/ml)." 
The top of the normal range for prolactin is usually around 15, and so these are clearly big swings. This isn't the first time that we've seen that on the forum. What would cause these kind of wild swings in prolactin? I believe that the root issue may be fluctuations in thyroid hormone output, and below I cite some evidence that this may be the case. In fact, this man had had four TSH reads, and three of them were over the new hypothyroidism threshold of 3.0 mUI/l. He did have a read of 2.2, but many alternative practitioners consider 2.0 to be the true cutoff for hypothyroidism. The bottom line is that much of his history indicates low thyroid function.
NOTE: High prolactin can lead to lowered dopamine, lowered libido, occasional gynecomastia and lowered testosterone levels. See my page on Testosterone, Prolactin and Prolactinomas for some information. And keep in mind that elevated prolactin should always be discussed with a doctor as some tumors (prolactinomas) secrete prolactin.
So let's look at the research evidence for the same thing - that hypothyroidism can cause elevated prolactin and just why this occurs. Here are a few studies that show just this:
1. Several Hypothyroid Disorders Linked Very Early to High Prolactin. Patients and physicians were reporting that often low thyroid function was linked to hyperprolactinemia (high prolactin). Furthremore, physicians had noted that this occurred frequently with several medical conditions:
"Two clinical syndromes have now been clearly shown to relate primary hypothyroidism (and therefore probably TRH hypersecretion) with excess prolactin secretion. These are the association of galactorrhea, amenorrhea, and primary hypothyroidism in post-pubertal individuals and of precocious puberty, glactorrhea and primary hypothyridism in children." 
2. 1978 Study on Men and Women with Primary Hypothyroidism. Notice this statement in one study from the same time period:
"The serum prolactin level was found to be elevated (>14.0 ng/ml) in 39 per cent of patients with untreated primary hypothyroidism, none of whom were receiving drugs known to affect serum prolactin levels." 
Clearly, that seem just a bit beyond coincidental, eh? (However, this study did not find nearly as significant of a link with male patients. As I'll show below, this is likely still significant for many men.) It would take years before resarchers discovered the mechanism underlying this correlation. Did hypothyroidism cause elevated prolactin or the other way around?
3. 2011 Study on Subclinical Hypothyroid Patients. Probably the most important population one could examine are those with subclinical hypothyrodism, defined as men and women with TSH < 10.0 mui/l,, since we get very few men on the Peak Testosterone Forum who have a TSH over 10. On the hand, low grade hypothyrodism, treated and untreated, is very common. This is the conidition that one 2011 study looked at and found:
"Sixty-two (13%) patients were males and 419 (87%) were females. The mean age of the patients was 32.53 10.13 years. Ninty-eight patients (91 females 7 males) had high prolactin. Prevalence of hyperprolactinemia in subclinical hypothyroidism was 20.4%. (11% in men and 22% in women)." 
MECHANISM OF ACTION: So which is the chicken and which is the egg? It turns out that TRH stimulates both TSH and prolactin. TRH is the upstream hormone secreted by the hypothalamus that in turn triggers the pituitary to secrete TSH which in turn tells the thyroid to produce T3 and T4. So clearly any type of hypothyroidism that would lead to elevated TRH levels would likely cause increased prolactin. And this is indeed the case generally with primary hypothyroidism for example.
Finally, let's look at the original case mentioned above where the man had wild swings in prolactin on the Peak Testosterone Forum. Could this man be experiencing some kind of primary hypothyroidism? His antibody counts were negative, so it would not appear to be Hashimoto's. There is one other fairly common cause of prmary hypothyroidism in men: an iodine deficiency. This actually does occur suprisingly often according to several studies. One study on school girls in the U.K. found that "urinary iodine measurements indicative of mild iodine deficiency were present in 51% of participants, moderate deficiency in 16%, and severe deficiency in 1%."  (Children are usually studied, because an iodine deficiency lowers IQ.)
Therefore, if you are seeing big changes in prolactin, it may be prudent to talk to a (knowledgeable) practitioner in thyroid hormones and an endocrinologist (in case of a tumor) as well. (CAUTION: Be very careful with supplementing iodine: it can lead to improved symptoms for a few months and then a severe crash of your thyroid hormones. Dosage is important and co-supplementing with selenium as well.)
NOTE: You may also be interested in my page on How to Lower Prolactin Levels as well.
2) Amer J of Medicine, May 1978, 64(5):782 787, "Serum prolactin levels in untreated primary hypothyroldism"
3) Prolactin: Physiology and Clinical Significance, 1973, D.F. Horrobin, p. 133.
4) Caspian J Intern Med. 2011 Spring; 2(2): 229 233, "Hyperprolactinemia in association with subclinical hypothyroidism"
5) Lancet, 2011 Jun 11, 377(9782):2007-12, "Iodine status of UK schoolgirls: a cross-sectional survey"