Women with infertility are routinely screened for thyroid dysfunction. A serum TSH level at the lower end of the acceptable range (2.5-4) is the goal. When the decision is made to provide thyroid supplementation, some debate exists about whether synthetic thyroid hormone, T4 (e.g. Synthroid/levothyroxine) is better than animal-source desiccated thyroid, which contains T3.
This article helps to provide some background information about why doctors choose T4 (levothyroxine) as the best choice for pregnant women or those planning to conceive.