Hyperprolactinemia is one of the most common endocrine disorders in the hypothalamic- pituitary axis. Although the most common etiologies of hyperprolactinemia are iatrogenic and prolactin producing adenoma (prolactinoma) — Another possible cause is primary hypothyroidism.
HOW? Secretion of Prolactin is primarily controlled by prolactin inhibitory hormone (dopamine) from the hypothalamus, however other factors [i.e VIP and Thyroid releasing hormone (TRH)] can influence its release.
TRH is a hypothalamic tripeptide that sits atop the hypothalamic/pituitary/thyroid axis; its primary function is stimulating release of thyrotropin (TSH) from the anterior pituitary gland. However it can also cause an increase in prolactin (PRL) secretion!
REFERENCES
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Hinkle et al. Desensitization, trafficking, and resensitization of the pituitary thyrotropin-releasing hormone receptor. Front. Neurosci., 13 December 2012 | http://dx.doi.org/10.3389/fnins.2012.00180
Bahar A, Akha O, Kashi Z, Vesgari Z. Hyperprolactinemia in association with subclinical hypothyroidism . Caspian Journal of Internal Medicine. 2011;2(2):229-233.
Dragana J, Xiangbing W. Primary Hypothyroidism Associated with Hyperprolactinemia and Pituitary Macroadenoma. Thyroid Science 6 (10): CR1-4, 2011