Levothyroxine (L-T4) in Fertility

Levothyroxine (L-T4) is a synthetic form of thyroid hormone taken as a pill. Clinicians prescribe it when the thyroid gland produces too little hormone (hypothyroidism). They also prescribe it for a milder form, subclinical hypothyroidism. In that form, the TSH (thyroid-stimulating hormone) blood test is high, but other thyroid levels appear normal. The goal is to restore normal thyroid levels before conception and during pregnancy.

Pregnancy increases the body's need for thyroid hormone. In women already taking levothyroxine, that need can begin to rise as early as the fifth week. In a 2004 study, the dose had to increase in 17 of 20 pregnancies. The average increase was 47 percent in the first half of pregnancy.¹ For this reason, clinicians often raise the dose as soon as pregnancy is confirmed. They then repeat the TSH test to adjust it.

Some women have normal thyroid tests but carry thyroid antibodies (TPO antibodies). Whether these women benefit from levothyroxine remains debated. In a 2006 study of pregnant women with these antibodies, miscarriage occurred in 3.5 percent of treated women. It occurred in 13.8 percent of untreated women.² The 2019 TABLET trial found a different result. It gave levothyroxine or a placebo to 952 women with these antibodies and a history of miscarriage or infertility. Live birth rates were about the same in both groups, 37.4 and 37.9 percent.³ TABLET tested one medication alone in women with normal thyroid tests. It did not test a full workup that looks for and treats other causes.

Restorative clinicians look for the causes behind infertility and recurrent miscarriage. A thyroid evaluation often belongs in that search. A high TSH can be one correctable cause. NaProTechnology protocols, for example, start levothyroxine when TSH is high. They monitor the dose with repeated blood tests through pregnancy. They also aim for a narrower TSH range than general practice uses.