Should infertility patients have their thyroid level checked? Yes. However, whether physicians should treat subtle abnormalities of thyroid function remains controversial. Overt hypothyroidism (underactive thyroid) often will have significant impact on your chances of conception; but what happens if you have an elevated TSH (Thyroid Stimulating Hormone) level and normal free thyroxine (T4) levels, i.e. subclinical or non-overt hypothyroidism (SCH)?
Approximately 8% of reproductive aged women may have SCH (subclinical, non-overt hypothyroidism). Although antithyroid antibodies are not used for the diagnosis of SCH, they are often measured. Antithyroid antibodies are called antimicrosomal antibodies and may slowly destroy the thyroid gland. Elevated levels may indicate an increased risk of overt hypothyroidism later in life.
TSH levels go up to make a tired thyroid make thyroid hormones, T4, sometimes called thyroxine. Hence when the thyroid gland fails to produce enough thyroid hormone, TSH levels rise in an attempt to stimulate the gland.
Why do infertility doctors prefer to see your TSH levels below 2.5?
Because TSH levels increase naturally during pregnancy and the upper limits of normal is 2.5 in the first trimester.
Is there an increased risk of miscarriage if TSH levels are elevated?
There is fair evidence that a TSH level greater than 4 mIU/L during pregnancy is associated with higher chances of first trimester miscarriage, but insufficient evidence exists that TSH levels between 2.5 and 4 are associated with miscarriage.
Other than increased miscarriage risk is there any other reason to have your TSH level below 4?
Yes there is fair, but not definitive, evidence that a TSH level over 4 is associated with an increased risk of adverse neurodevelopmental outcomes and miscarriage.
Conclusion: Attempt to treat SCH to lower TSH levels below 2.5, but TSH levels below 4 are not associated with increased adverse fetal outcomes. Once alterations are made in thyroid supplementation, i.e. synthroid, levothyroxine, etc., it takes approximately three weeks to tell if increased dosing has lowered the TSH level. Therefore thyroid medication alterations should not be followed more frequently than monthly.