Normal thyroid hormone production is though to be important for reproduction.  A slightly underactive thyroid gland does not affect the success of IVF according to a recent study that was presented at the annual meeting of the American Society for Reproductive Medicine in 2007.

Thyroid hormone production is tightly regulated by the body. Thyroid hormone production which is too low is called hypothyroidism . Thyroid hormone production that is too high is called hyperthyroidism. The most accurate way to determine the status of the thyroid is by using a blood called TSH or thyroid stimulating hormone. A TSH level that is too high is indicative of hypothyroidism. TSH levels are a very sensitive way to determine thyroid status. In many cases, women can be found with slight increases in their TSH level but without any of the other signs or symptoms of hypothyroidism. This is called sub-clinical hypothyroidism.
Doctors have wondered whether sub-clinical hypothyroidism affects the success of in vitro fertilization cycles. A study was presented in which a large group of women had their TSH levels checked a few weeks before starting an IVF cycle. A normal TSH level was considered to be between 0.4 and 4.0.
A total of 723 women were studied. Twenty seven of the, or 3.7%, were found to have an elevated TSH level over 4.0. The highest level was 11.10.  The remaining 696 women had normal TSH levels.
There was no difference found in the number of eggs that were retrieved . There was also no difference in the percentage of eggs that became fertilized or the number of embryos that were transferred into the women’s uterus.
Most importantly, there was no difference in the pregnancy rates.
Some doctors believe that the current upper limit for TSH of 4.0 is too high and that the normal levels should be adjusted. In order to address this issue, the doctors in this study reanalyzed the data using a TSH level of 2.5 as a cutoff.
In this analysis, there were 127 women with an elevated TSH. however, there were still no differences between the hypothyroid group and the other women in the study.
This study shows us that subtle hypothyroidism does not have an effect on the success of IVF.