Low Prolactin is NOT Caused by Low Vitamin D
No, low prolactin levels are not caused by low vitamin D levels. The available evidence demonstrates no causal relationship between vitamin D deficiency and hypoprolactinemia (low prolactin). In fact, the research shows the opposite association: low vitamin D is linked to elevated prolactin levels, not low ones.
The Evidence Shows the Opposite Relationship
The relationship between vitamin D and prolactin actually works in reverse of what the question suggests:
Low vitamin D is associated with HIGH prolactin levels, not low prolactin. Women with prolactinomas (prolactin-secreting tumors) have significantly lower vitamin D levels compared to controls, and those with larger tumors have even lower vitamin D levels 1.
In patients with macroprolactinemia (elevated macroprolactin), vitamin D supplementation (2,000 IU daily for 4 months) actually reduced total prolactin and macroprolactin levels, suggesting that low vitamin D may contribute to elevated prolactin 2.
Chronic hypocalcemia (which can occur with vitamin D deficiency) does not inhibit prolactin secretion or affect basal prolactin levels 3.
Studies in humans found that elevated prolactin levels have no effect on vitamin D metabolism or calcium absorption, indicating prolactin does not regulate vitamin D, and by extension, vitamin D does not regulate prolactin in a clinically meaningful way 4.
Actual Causes of Low Prolactin
When evaluating hypoprolactinemia, consider these established etiologies:
Pituitary damage or dysfunction is the primary cause, often occurring as part of combined pituitary hormone deficiencies rather than isolated prolactin deficiency 5.
Hypopituitarism from various causes including pituitary tumors (particularly non-secreting adenomas), pituitary surgery, radiation, or infiltrative diseases should be evaluated by assessing other pituitary hormones 6, 5.
Isolated prolactin deficiency is rare but can occur 5.
Diagnostic Approach for Low Prolactin
Measure basal prolactin levels using gender-specific cut-offs: at least 5 ng/mL for males and 7 ng/mL for females can be used to define normal prolactin reserve 5.
Consider TRH stimulation testing if hypoprolactinemia is suspected: minimum peak prolactin responses of 18 ng/mL for males and 41 ng/mL for females exclude hypoprolactinemia 5.
Evaluate for other pituitary hormone deficiencies, particularly in cases of suspected hypopituitarism, as low prolactin may serve as a marker for extensive pituitary gland damage 6, 5.
Clinical Significance of Low Prolactin
Low prolactin may contribute to infertility and menstrual irregularities in women 6.
Prolactin has anxiolytic properties, and low levels may reduce stress buffering and are associated with increased risk of postpartum depression symptoms 6.
Regular follow-up is recommended for patients with persistent hypoprolactinemia, particularly if associated with other pituitary hormone deficiencies 6.
Common Pitfall to Avoid
Do not attribute low prolactin to vitamin D deficiency, as this relationship is not supported by evidence. Instead, focus on evaluating for pituitary pathology and other hormone deficiencies when hypoprolactinemia is identified.