The thyroid is a gland in your neck that produces hormones in both women and men. It has many effects in the body, and can also effect fertility.
We test the thyroid using simple bloodwork and look at the following parameters:
TSH - thyroid stimulating hormone. This is a hormone that comes from your brain to tell your thyroid to work. We would like your TSH < 2.0 when trying to conceive
Free T3 - This is a hormone produced (using iodine) by the thyroid gland that circulates in the blood largely bound to a protein called the Thyroxin Binding Globulin. That being said, it is the T3 that is NOT bound that we find to be biologically active.
Free T4 - This is the main hormone produced (using Iodine) by the thyroid gland . It is much more biologically active if it's converted (using selenium) into T3. Like T3, the T4 circulating in your blood that is NOT bound to a protien is more biologically active.
Anti-Thyroid Peroxidase Antibodies - Anti-TPO Ab are present either when your thyroid function is abnormal and the cause is auto-immune or when there may be a future risk of thyroid disease. Graves disease and Hashimoto's thyroiditis are associated with the presence of Anti-TPO Ab. They tell us that your body is attacking your thyroid is some way.
With respect to fertility - having abnormal thyroid levels can make it harder to conceive and carry to term. There is also some theories that simply having Anti-TPO antibodies can make it harder to concieve and carry to term, though this is more controversial.
Pharmaceuticals are really good at getting your T3, T4, and TSH levels in check. Once pregnant the thyroid can go into a state of flux so must be monitored with bloodwork in anyone who is already on thyroid medication. I have found in my practice that Anti-TPO antibodies are somewhat ignored in the fertility world as long as your TSH levels are in check. I believe that it's better to have the Anti-TPO Ab levels within the normal range as well and will work with patients to get them there. This may include selenium supplementation, a gluten-free diet, and other herbs to work on the body's immune response.