Most practice guidelines on managing thyroid disease during pregnancy focus on treatment during gestation. However, to promote a healthy pregnancy and reduce patient anxiety, it’s essential to optimize thyroid conditions during the preconception period.
Thyroid hormone requirements typically increase by 30%-50% during pregnancy, beginning in early pregnancy and coinciding with organogenesis — often before patients are even aware of their pregnancy. Alarmingly, studies have found that about half of patients with hypothyroidism may have thyroid-stimulating hormone (TSH) levels outside recommended pregnancy reference ranges in early pregnancy. Effective preconception counseling aims to achieve a euthyroid state for both mother and fetus from early pregnancy through delivery.
Iodine
Iodine requirements also increase during pregnancy and lactation (to 250 µg daily). While the US population generally consumes sufficient iodine, women may experience mild to moderate iodine deficiency during pregnancy. Patients who can’t meet their iodine needs through diet may require supplementation with potassium iodide at 150 µg daily, starting 3 months before conception.
Because most Americans obtain iodine from seafood, dairy, and iodized salt, individuals on restricted diets — such as plant-based, vegan, or those using non-iodized salts like Himalayan or sea salt — may be at risk for deficiency. Therefore, clinicians should inquire about dietary restrictions during preconception counseling.
Overt Hypothyroidism
Patients with overt hypothyroidism should be advised to increase their levothyroxine dose by 20%-30% upon becoming pregnant. For example, a patient taking one tablet of levothyroxine daily can adjust their regimen to take one tablet daily except on Monday and Thursday, for example, when they would take two tablets. Patients should be advised to contact their clinician immediately once pregnant for thyroid function testing.
Prior to conception, maintaining TSH levels between 0.5 and 2.5 mIU/L reduces the risk for elevated TSH in early pregnancy. Patients should be instructed to separate levothyroxine from any iron or calcium supplements by at least 4 hours to prevent absorption interference. If they experience frequent morning vomiting once pregnant, they can switch their levothyroxine dosing to nighttime, ensuring that at least 2 hours have passed since their last meal.
Additionally, patients with a history of thyroid cancer and subsequent hypothyroidism should follow similar dose adjustment guidelines to achieve the same degree of TSH suppression during pregnancy as they had preconception.
T4 Needed for Fetal Development
When planning for pregnancy, patients using thyroid hormone formulations other than levothyroxine, such as desiccated thyroid hormone (which contains both T4 and T3) or liothyronine (T3), should be transitioned to levothyroxine (T4). This is important because T4 can cross the fetal blood-brain barrier, while T3 cannot.
During the first half of pregnancy, the fetus is unable to produce sufficient thyroid hormone and relies on maternal thyroid hormone for development. That means it is essential for patients to switch to levothyroxine and optimize TSH levels prior to conception.
Subclinical Hypothyroidism
Subclinical hypothyroidism management (characterized as elevated TSH with normal T4) during preconception and pregnancy remains a topic of debate. Some guidelines (here and here) suggest that no treatment or monitoring is necessary before conception, including with assisted reproductive technology (ART).
However, it may be advisable to treat persistent cases of subclinical hypothyroidism prior to conception with a low dose of levothyroxine (25-50 µg daily) to maintain TSH levels between 0.5 and 2.5 mIU/L. This approach is reasonable given the increased thyroid hormone requirements during pregnancy.
Thyroid Autoimmunity
Patients with thyroid autoimmunity (TPO Ab + or TgAb+) exhibit a diminished response to the maternal hormone beta-hCG during pregnancy and are at an increased risk of progressing to subclinical or overt hypothyroidism, both preconception and during pregnancy. Therefore, it is advisable to monitor thyroid function periodically while patients are actively trying to conceive and to treat persistent subclinical or overt hypothyroidism.
Recent randomized controlled trials (here, here, and here) of levothyroxine intervention on miscarriages and live births have not demonstrated a benefit of levothyroxine treatment in patients who are euthyroid and remain euthyroid with TPO Ab+ prior to conception.
Assisted Reproductive Technology
Patients undergoing ART, including intrauterine insemination and in vitro fertilization, should have their thyroid function assessed before ovarian stimulation and embryo implantation. Both subclinical and overt hypothyroidism ideally should be treated with levothyroxine, and TSH levels optimized prior to initiating ART. Monitoring thyroid function during ART can be challenging as ovarian stimulation may cause transient abnormalities in thyroid function tests secondary to the medications used in the process.
Clinicians can optimize thyroid function during pregnancy by providing comprehensive preconception counseling. While treatment for overt hypothyroidism is clear, a more nuanced discussion is necessary for those with subclinical hypothyroidism and thyroid autoimmunity.
COMMENTARY
Optimize Thyroid Before and After Pregnancy
DISCLOSURES
| November 06, 2024Editorial Collaboration
&
Most practice guidelines on managing thyroid disease during pregnancy focus on treatment during gestation. However, to promote a healthy pregnancy and reduce patient anxiety, it’s essential to optimize thyroid conditions during the preconception period.
Thyroid hormone requirements typically increase by 30%-50% during pregnancy, beginning in early pregnancy and coinciding with organogenesis — often before patients are even aware of their pregnancy. Alarmingly, studies have found that about half of patients with hypothyroidism may have thyroid-stimulating hormone (TSH) levels outside recommended pregnancy reference ranges in early pregnancy. Effective preconception counseling aims to achieve a euthyroid state for both mother and fetus from early pregnancy through delivery.
Iodine
Iodine requirements also increase during pregnancy and lactation (to 250 µg daily). While the US population generally consumes sufficient iodine, women may experience mild to moderate iodine deficiency during pregnancy. Patients who can’t meet their iodine needs through diet may require supplementation with potassium iodide at 150 µg daily, starting 3 months before conception.
Because most Americans obtain iodine from seafood, dairy, and iodized salt, individuals on restricted diets — such as plant-based, vegan, or those using non-iodized salts like Himalayan or sea salt — may be at risk for deficiency. Therefore, clinicians should inquire about dietary restrictions during preconception counseling.
Overt Hypothyroidism
Patients with overt hypothyroidism should be advised to increase their levothyroxine dose by 20%-30% upon becoming pregnant. For example, a patient taking one tablet of levothyroxine daily can adjust their regimen to take one tablet daily except on Monday and Thursday, for example, when they would take two tablets. Patients should be advised to contact their clinician immediately once pregnant for thyroid function testing.
Prior to conception, maintaining TSH levels between 0.5 and 2.5 mIU/L reduces the risk for elevated TSH in early pregnancy. Patients should be instructed to separate levothyroxine from any iron or calcium supplements by at least 4 hours to prevent absorption interference. If they experience frequent morning vomiting once pregnant, they can switch their levothyroxine dosing to nighttime, ensuring that at least 2 hours have passed since their last meal.
Additionally, patients with a history of thyroid cancer and subsequent hypothyroidism should follow similar dose adjustment guidelines to achieve the same degree of TSH suppression during pregnancy as they had preconception.
T4 Needed for Fetal Development
When planning for pregnancy, patients using thyroid hormone formulations other than levothyroxine, such as desiccated thyroid hormone (which contains both T4 and T3) or liothyronine (T3), should be transitioned to levothyroxine (T4). This is important because T4 can cross the fetal blood-brain barrier, while T3 cannot.
During the first half of pregnancy, the fetus is unable to produce sufficient thyroid hormone and relies on maternal thyroid hormone for development. That means it is essential for patients to switch to levothyroxine and optimize TSH levels prior to conception.
Subclinical Hypothyroidism
Subclinical hypothyroidism management (characterized as elevated TSH with normal T4) during preconception and pregnancy remains a topic of debate. Some guidelines (here and here) suggest that no treatment or monitoring is necessary before conception, including with assisted reproductive technology (ART).
However, it may be advisable to treat persistent cases of subclinical hypothyroidism prior to conception with a low dose of levothyroxine (25-50 µg daily) to maintain TSH levels between 0.5 and 2.5 mIU/L. This approach is reasonable given the increased thyroid hormone requirements during pregnancy.
Thyroid Autoimmunity
Patients with thyroid autoimmunity (TPO Ab + or TgAb+) exhibit a diminished response to the maternal hormone beta-hCG during pregnancy and are at an increased risk of progressing to subclinical or overt hypothyroidism, both preconception and during pregnancy. Therefore, it is advisable to monitor thyroid function periodically while patients are actively trying to conceive and to treat persistent subclinical or overt hypothyroidism.
Recent randomized controlled trials (here, here, and here) of levothyroxine intervention on miscarriages and live births have not demonstrated a benefit of levothyroxine treatment in patients who are euthyroid and remain euthyroid with TPO Ab+ prior to conception.
Assisted Reproductive Technology
Patients undergoing ART, including intrauterine insemination and in vitro fertilization, should have their thyroid function assessed before ovarian stimulation and embryo implantation. Both subclinical and overt hypothyroidism ideally should be treated with levothyroxine, and TSH levels optimized prior to initiating ART. Monitoring thyroid function during ART can be challenging as ovarian stimulation may cause transient abnormalities in thyroid function tests secondary to the medications used in the process.
Clinicians can optimize thyroid function during pregnancy by providing comprehensive preconception counseling. While treatment for overt hypothyroidism is clear, a more nuanced discussion is necessary for those with subclinical hypothyroidism and thyroid autoimmunity.
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