Thyroid function and IVF outcome: when to investigate and when to intervene?

To summarize the evidence regarding the need to assess thyroid function in women undergoing ovarian stimulation as well as the need to intervene when thyroid function is suboptimal.

There is now evidence that ovarian stimulation can influence thyroid function not only via hyperestrogenism but also directly, since thyroid-related receptors are present in human granulosa cells and in the endometrium. Prospective and retrospective observational studies, as well as a few clinical trials, have been conducted in an effort to clarify the association between ovarian stimulation and thyroid function with controversial results.

The need of thyroid function screening with thyroid stimulating hormone (TSH) in infertile women attempting pregnancy is recognized by many international societies. Since TSH is a simple, cheap screening tool and levothyroxine (LT4) supplementation is an easy to apply, cheap and well tolerated intervention, universal thyroid screening in women undergoing IVF represents a reasonable policy. In case of subclinical hypothyroidism, when TSH exceeds the threshold of 4.0 or 4.5 μIU/ml before IVF, LT4 replacement should be administered, while the same intervention might also be justified for women with TSH concentration more than 2.5 μIU/ml before IVF.