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Information on the latest vitamin D news and research.

Find out more information on deficiency, supplementation, sun exposure, and how vitamin D relates to your health.

New RCT: Vitamin D helps with endometrial thickness in PCOS

A new randomized controlled trial has found vitamin D improves endometrial thickness in women with polycystic ovary syndrome.

Polycystic ovary syndrome (PCOS) affects more than 5% of women in childbearing age. PCOS is a disorder characterized by failure to ovulate, affecting the regularity of menstrual cycle, ability to have children, and hormone balance. It can also affect the heart, blood vessels, and lead to changes in appearance (women with PCOS can appear slightly masculine). We have covered this condition in our blog before.

A major problem in PCOS is difficulty in becoming pregnant. Under normal circumstances, when a woman ovulates, the follicles in her ovaries grow and fill with necessary hormones for the egg to mature before it is released.

Pregnant or not, at the end of the ovulation cycle the follicle bursts, releasing the egg into the fallopian tube where it travels to the uterus for fertilization. In PCOS, the follicles don’t receive adequate hormones and subsequently don’t burst, leaving a buildup of follicles (or cysts) in the ovaries.

In the present study, researchers wanted to examine the effect of vitamin D on overall ability for women with PCOS to become pregnant using intrauterine insemination (IUI), a form of artificial fertilization that is not considered an artificial reproduction technique (ART). This procedure is common for women with a variety of conditions who are trying to become pregnant but have barriers doing so.

During ovulation, the lining of the uterus (the endometrium) naturally thickens in order to prepare for the egg and its growth and to prepare for pregnancy. A thin endometrium can cause infertility, and women with PCOS often have problems with endometrial thickness. One reason is that they often take a drug called Clomid, which helps with balancing their hormones. A side effect of this drug is thinning of the endometrium, which can hurt your chances of becoming pregnant.

Previous research has found that there are vitamin D receptors in the endometrium. Thus, the researchers specifically wanted to study how vitamin D affects endometrial thickness.

Researchers conducted this trial on 110 PCOS patients having difficulty becoming pregnant and about to undergo IUI. The group was randomized, half receiving a shot of 300,000 IU of vitamin D and the other half receiving a shot of placebo.

After two months, the women in both groups received the IUI protocol. Researchers measured endometrial thickness and pregnancy rates of all the women in the study after 12 weeks from the start of their IUI protocol.

Here’s what the researchers found:

  • Both placebo and vitamin D groups were generally deficient to start the trial, with no statistical difference between the two (started around 12 ng/ml).
  • At the end of the trial, endometrial thickness in the vitamin D group was 8.70mm, as compared to 7.59mm in the placebo group. This was statistically significant.
  • Rates of pregnancy after 12 weeks in the vitamin D and placebo groups were 38.2% and 32.7%, respectively. Thus, vitamin D supplementation increased probability of becoming pregnant by about 17%. However, this was not statistically significant.

The researchers concluded,

“If endometrial thickness proves to be responsible for IUI or other ART outcomes, administration of vitamin D would be a promising option to compensate and reverse the adverse effects of clomiphene citrate (Clomid) on endometrial thickness.”

The trial was limited by its small sample size. The researchers call for a study with a larger sample size to better evaluate the effect of vitamin D on IUI pregnancy.


Asadi, M. et. al. Vitamin D improves endometrial thickness in PCOS women who need intrauterine insemination: a randomized double-blind placebo-controlled trial. Archives of Gynecology and Obstetrics, 2013.

  About: Jeff Nicklas

Jeff Nicklas was a staff member for the Vitamin D Council from October 2013 to January 2015. He is now pursuing his passion for public health through graduate studies.

One Response to New RCT: Vitamin D helps with endometrial thickness in PCOS

  1. John says:

    Wow, very impressive for a SINGLE shot of vitamin D but let’s see 5-10K daily after this dosage. I can only imagine that a daily intake capable of increasing vitamin D levels to optimal levels would achieve better results. btw, I would recommend that you add “fertility” and “infertility” to your key words. Information like this can help all couples wanting to start a family.