Physiologic Changes
What hormone causes suppression of TSH in early pregnancy due to weak TSH receptor stimulation?
Answer: hCG – during first 12 weeks of gestation.
hCG stimulates thyroid hormone receptors (which increases free T4), which suppresses hypothalamic thyrotropin releasing hormone that in turn limits pituitary TSH secretion.
What is the most common cause of overt hyperthyroidism in pregnancy?
Answer: Graves disease (≈95%)
The antibodies involved in Graves disease during pregnancy are thyroid-stimulating hormone receptor antibodies (TRAb).
What is the most common cause of hypothyroidism in pregnancy?
Answer: Hashimoto thyroiditis.
Characterized by glandular destruction by autoantibodies - thyroid peroxidase (TPO) antibodies and thyroglobulin (Tg) antibodies.
How is subclinical hypothyroidism defined in pregnancy?
Answer: Elevated TSH with normal free T4.
What postpartum condition causes transient hyper- then hypothyroidism within 12 months of delivery?
Answer: Postpartum thyroiditis
Presentation:
By approximately how much does maternal thyroid volume increase during pregnancy?
Answer: 10–30% - primarily during the third trimester.
Attributable to increases in extracellular fluid and blood volume during pregnancy.
Thyroid function testing is not recommended in asymptomatic pregnant patients with mild thyroid enlargement.
What two classic physical exam findings are specific to Graves disease?
Answer: Ophthalmopathy (lid lag and lid retraction) & dermopathy (pretibial myxedema)
How is overt hypothyroidism defined in pregnancy based on labs?
Answer: Elevated TSH with low free T4.
Nonspecific clinical findings may include: fatigue, constipation, cold intolerance, muscle cramps, weight gain, prolonged relaxation phase of deep tendon reflexes, +/- goiter.
This thyroid condition is characterized by low TSH and normal free T4, and ACOG states it is not associated with adverse pregnancy outcomes.
Answer: Subclinical hyperthyroidism.
Why are thioamides ineffective in treating postpartum thyroiditis? (Describe the drug MOA as it relates to the pathophysiology of thyroiditis.)
Answer: Thioamides work to block hormone synthesis. Hyperthyroidism in thyroiditis is due to hormone release from gland destruction, not increased hormone synthesis, so it's not effective.
After what gestational age can nonpregnant TSH reference ranges generally be used?
Answer: After the first trimester.
Levels normalize afterward, allowing use of nonpregnant reference ranges.
What maternal complications are increased with inadequately treated hyperthyroidism? (Name at least 2)
Answer: Preeclampsia with severe features, heart failure, thyroid storm
What is the recommended daily iodine intake (mcg/day) for pregnant women?
Answer: 220 micrograms/day.
290 micrograms/day in lactating patients.
Majority of women in USA have adequate iodine supplementation.
Which patients should be tested for thyroid disease during pregnancy? (Name at least 3 categories)
Answer: Those with clinical suspicion for thyroid disease, a personal history of thyroid disease, type 1 diabetes, and/or family history of thyroid disease.
ACOG recommends against universal screening of all pregnant patients.
What medication class may be used to treat symptomatic hyperthyroidism in postpartum thyroiditis?
Answer: B-blocker, if symptoms are severe.
At how many weeks of gestation does the fetal thyroid gland begin concentrating iodine and synthesizing thyroid hormone?
Answer: Approx. 12 weeks of gestation.
Of note, maternal T4 is still transferred to fetus throughout entire pregnancy and necessary for normal fetal brain development.
Which antithyroid medication is avoided in the first trimester and why?
Answer: Methimazole - associated with embryopathy such as esophageal atresia, choanal atresia, aplasia cutis (congenital skin defect)
Propylthiouracil can be used in the first trimester.
What adverse pregnancy outcomes are associated with untreated overt hypothyroidism? (Name at least 3)
Miscarriage, preeclampsia, preterm birth, placental abruption, stillbirth.
Large randomized trials — including the CATS trial (2012) and the MFM Units Network trial (2017) — demonstrated that screening and treating this condition during pregnancy does not improve offspring neurocognitive outcomes.
Answer: Subclinical hypothyroidism.
These trials demonstrated no difference in neurocognitive development in offspring through age 5 who were born to women screened and treated for subclinical hypothyroidism.
Follow up on children from CATS study through age 9 years also confirmed no neurodevelopmental improvement in offspring of treated women.
What heart condition is heavily associated with thyroid storm?
Answer: Thyrotoxic heart failure.
Thyroid storm: hyper-metabolic state caused by excess thyroid hormone.
Signs/Symptoms: fever, tachycardia, cardiac dysrhythmia, CNS dysfunction.
Even if fetal status is not reassuring in acute setting of thyroid storm, status may improve as maternal status is stabilized. Should attempt to avoid delivery in presence of thyroid storm.
Approximately what percentage of thyroxine (T4) in umbilical cord blood at delivery is maternal in origin?
Answer: About 30%.
Maternal T4 continues to cross the placenta throughout pregnancy and makes up roughly one-third of cord blood T4 at delivery.
What fetal findings should raise concern for fetal thyrotoxicosis? (Name at least 2)
Answer: Persistent fetal tachycardia, growth restriction, fetal goiter, hydrops
Why should T3-containing thyroid preparations (desiccated thyroid or synthetic T3) be avoided in pregnancy?
Answer: T3 preparations of thyroid hormone should be avoided in pregnancy as high levels of T3 lead to supraphysiologic levels of maternal T3 and low levels of T4. Maternal T4 is critical for fetal CNS development.
Treatment of hypothyroidism: T4 replacement with Levothyroxine (starting dose 1-2 mcg/kg daily or approximately 1 mcg daily)
In which patient population might thyroid antibody testing influence fetal surveillance decisions?
Answer: Pregnant patients with Graves disease.
What percentage of women with postpartum thyroiditis eventually develop permanent hypothyroidism?
Answer: Approximately one-third.