Hyperthyroidism and Hyperprolactinemia

Is There Any Association?

Mojgan Sanjari, MD; Zohreh Safi, BS; Khatereh Mohammadi Tahroodi, MD


Endocr Pract. 2016;22(12):1377-1382. 

In This Article

Abstract and Introduction


Objective: To compare the serum prolactin level in hyperthyroid and normal control females. Hyperthyroidism is a common disease. Although a direct association has been demonstrated between hypothyroidism and increased prolactin levels, this association has not been established for hyperthyroidism.

Methods: Cross-sectional study in cases and control groups. Control subjects were chosen from those participating in the Kerman Coronary Artery Disease Risk Factors study. To select the cases, all women referred to the laboratories of Kerman with a thyroid-stimulating hormone (TSH) level ≤0.5 mIU/L who met the inclusion criteria were entered in the study. A total of 231 women aged 15 to 50 years were enrolled. The case group included 71 hyperthyroid women, and the control group included 160 women with normal thyroid function matched by age.

Results: The mean (SD) serum level of prolactin was 16.56 (0.97) ng/mL (95% confidence interval [CI], 15.41 ng/mL to 15.71 ng/mL) in the controls and 23.07 (1.49) ng/mL (95% CI, 22.7 ng/mL to 23.4 ng/mL) in the case subjects. Hyperprolactinemia was more common in the hyperthyroid group (16.5 [0.97] ng/mL versus 23.07 [1.49] ng/mL; P<.001). The prolactin level decreased with age. Hyperthyroidism and estradiol increased the prolactin level. After adjusting for age and estradiol, hyperthyroidism increased the serum prolactin level (P<.001).

Conclusion: The results of this study revealed that hyperprolactinemia is more frequent in hyperthyroid females. Serum prolactin level can be increased in hyperthyroidism.


Prolactin (PRL) is a lactogenic hormone secreted by the anterior pituitary. Hyperprolactinemia is a case where the baseline or fasting level of PRL in the morning and in the absence of pregnancy and lactation is >25 ng/mL [1]. PRL levels begin to increase due to a variety of reasons, including the following:

  1. Physiologic reasons (sleep, exercise, physical stress, emotional stress, breast stimulation, and high-protein diet);

  2. Use of certain medications, such as estrogen, neuroleptic drugs, metoclopramide, antidepressants, cimetidine, methyldopa, reserpine, verapamil, risperidone);

  3. In hypothyroid patients and patients with chronic kidney and liver diseases;

  4. In patients with prolactinomas, destructive lesions of hypothalamus or pituitary stalk.

Therefore, in the study of hyperprolactinemia, drug causes and diseases such as hypothyroidism should be considered.[1]

It has been reported that hypothyroidism can cause an increase in PRL by increasing the thyrotropin-releasing hormone (TRH) level.[1] However, the direct effect of hormones on serum PRL levels and the relationship between hyperthyroidism and PRL is unclear,[2,3] as thyroxine (T4) did not alter prolactin level in other studies.[4,5] Still, PRL levels in chronic hyperthyroidism was found to be normal or slightly above normal.[6,7] Although PRL production increases in hyper- and hypothyroidism, due to increased clearance in hyperthyroidism, an elevated blood level of PRL was found in patients with hypothyroidism.[8] Baseline PRL levels in hyperthyroid patients were higher than in normal subjects.[9] One of the important points regarding thyroid hormones is their contribution in the regulation of pituitary sensitivity and function, as receptors of the pituitary gland surface are regulated by thyroid hormones. Accordingly, thyroid hormones regulate the number of TRH receptors on anterior pituitary mammotrophs.[2]

A significant relationship has been demonstrated between autoimmunity and decreased levels of PRL in autoimmune diseases.[10] PRL secreted by the anterior pituitary and prolactin-like polypeptides that are localized in the joints are effective in modulating the chondrogenic differentiation of synovial cell function, and there is mild hyperprolactinemia in patients with rheumatoid arthritis.[11] The study of Parker and colleagues[12] on the relationship between PRL and autoimmune diseases showed that PRL levels are increased in autoimmune diseases such as lupus. Interestingly, elevated blood levels of PRL have been shown in patients with Hashimoto disease.[13]

Based upon our observations and experience in which PRL and thyroid tests are requested for patients with menstrual cycle disorders, when the patient is hyperthyroid, the patient's PRL is high and returns to its normal level after treatment for hyperthyroidism. As hyperprolactinemia needs further expensive evaluation, such as magnetic resonance imaging of the pituitary gland, based on a literature review and to the best of our knowledge, there are only a few studies that have been conducted on the levels of PRL in women with hyperthyroidism, and the results have been contradictory.[4–9] Thus, the present study was designed to compare the PRL levels in healthy women and patients with hyperthyroidism, because proving any association between hyperprolactinemia and hyperthyroidism may lead to postponement of requests for time-consuming and costly procedures.