Plan surveillance
- Antithyroid-drug use
- TRAb/TSI >3× ULN
- Uncontrolled hyperthyroidism
OpenMFM Clinical Education
Fetal thyroid assessment in maternal thyroid disease
Should we evaluate the neck? Yes, selectively.
PhysiciansAdvanced practice providersMFM sonographers
Evidence reviewed August 24, 2026 · Dr. Chukwuma Onyeije · OpenMFM.org
Opening case
A treated Graves history can remain biologically active
Maternal statusEuthyroid on levothyroxine
HistoryRadioactive iodine for Graves disease
UltrasoundNormal detailed anatomy at 19 weeks
Missing dataTRAb / TSI
Does this fetus need targeted thyroid surveillance?
Guideline-backed
Learning objectives
Identify. Image. Interpret. Escalate.
Identifywho needs surveillance
→
Imagethe gland and systems
→
Interpretthe composite phenotype
→
Escalatewhen management changes
The goal is not to “see the neck.” The goal is to detect fetal thyroid dysfunction early enough to change care.
The practical answer
The answer is yes, selectively
Investigate now
- Fetal goiter
- Persistent tachycardia
- Growth/cardiac/fluid phenotype
Routine pathway
- Stable Hashimoto disease
- Levothyroxine only
- No Graves history or fetal signal
Guideline-backed
Mechanism
Two placental pathways create opposite fetal phenotypes
Maternal antibody
TRAb / TSI
Persists after thyroidectomy or radioactive iodine.
Maternal therapy
PTU · methimazole · carbimazole
Crosses the placenta and suppresses fetal hormone synthesis.
stimulates
suppresses
Fetal hyperthyroidism
Stimulation may continue even when the mother is hypothyroid on replacement therapy.
Fetal hypothyroidism
Medication effect can enlarge the gland and impair fetal thyroid function.
Indications
Risk is defined by exposure, control, and phenotype
Maternal history
Current or past Graves disease, including definitive treatment.
Laboratory / medication
TRAb/TSI >3× ULN, antithyroid drugs, or excess iodine exposure.
Targeted fetal thyroid surveillance
Maternal control
Uncontrolled overt hyperthyroidism in the second half of pregnancy.
Fetal phenotype
Goiter, tachycardia, FGR, cardiomegaly, hydrops, abnormal maturation, or polyhydramnios.
Guideline-backed
Graves pathway
Surveillance begins with antibodies, not ultrasound
First trimester
Every current or prior Graves case: TSH, FT4, and TRAb/TSI.
18–22 weeks
If initial TRAb/TSI is >3× ULN, repeat antibodies and begin fetal thyroid surveillance.
30–34 weeks
Repeat sustained elevation to guide late fetal and neonatal monitoring.
Stop rule
If TRAb/TSI is below 3× ULN and the mother remains euthyroid, follow-up antibody testing and fetal hyperthyroidism monitoring can stop.
Medication exposure
Antithyroid drugs require a fetal thyroid plan
Maternal treatment must control disease without suppressing the fetal gland.
18–20
weeks: start fetal thyroid ultrasound
Monthly
typical surveillance cadence
Frequency may be reduced for low-dose treatment after individualized review.
Current guidance is stronger than “consider imaging.”
All pregnant patients using antithyroid drugs require a fetal thyroid monitoring plan.
Guideline-backed
Avoid over-imaging
Well-controlled Hashimoto disease does not justify serial neck imaging
Stable hypothyroidism
Normal maternal thyroid function on levothyroxine
→
Standard anatomy evaluation
Routine fetal neck survey for masses
→
No serial thyroid imaging
Unless another risk modifier appears
Graves history
TRAb/TSI risk
ATD / iodine exposure
Fetal phenotype
TPO antibodies identify maternal autoimmunity. They do not create the same fetal-stimulation pathway as TRAb/TSI.
Sonography technique
Find the gland in the true transverse neck plane
Level
Below the mandible and above the clavicles.
Landmarks
Center the trachea; identify paired carotid vessels.
Gland
Document both lobes anterior and lateral to the trachea.
Plane
Avoid obliquity before measuring circumference or area.
Imaging standard
Thyroid biometry
Measure size against gestational age, not visual impression
Thyroid circumference
Gestational age or BPD
95th percentile
50th
90–95th
percentile supports thyromegaly
Record the reference
Use a validated gestational-age or BPD-based nomogram and name it in the report.
Size does not define function
A large thyroid may be hyperthyroid or hypothyroid.
Targeted examination
A thyroid scan is a fetal systems examination
Fetal thyroid
assessment
assessment
Gland size · shape · echogenicity
Vascular distribution
Airway · neck position · swallowing
Heart rate · cardiac function · hydrops
Growth · movement
Fluid · bone maturation
Interpretation
Goiter proves enlargement, not thyroid direction
Possible pathway
Fetal hyperthyroidism
Stimulating TRAb/TSI may enlarge and activate the gland.
Fetal
goiter
goiter
Possible pathway
Fetal hypothyroidism
ATD or iodine suppression may enlarge an underfunctioning gland.
The correct next question is not “Is there a goiter?” It is “What is this gland doing?”
Composite phenotype
The phenotype, not one Doppler pattern, guides the diagnosis
| Feature | Hyperthyroidism favored | Hypothyroidism favored |
|---|---|---|
| Vascularity | Central / diffuse | Peripheral or reduced |
| Heart rate | Persistent >160–180 bpm, relatively late | Usually normal; bradycardia is not required |
| Bone maturation | Accelerated, distal femoral center <31 weeks | Delayed |
| Movement | Increased | Reduced |
| Growth / cardiac | FGR, cardiomegaly, failure, hydrops | Often less specific |
| Mechanical effect | Goiter may occur | Neck extension, tracheal compression, polyhydramnios |
Overlap is common. Maternal antibodies, medication dose, and serial change remain part of the diagnosis.
Doppler pitfalls
Doppler is a clue with technical traps
Interpretable distribution
Blooming / motion artifact
Use a low velocity scale carefully
Fetal motion can create false central flow.
Apply ALARA
Minimize output and dwell time. Never classify function from color Doppler alone.
Differential diagnosis
Not every anterior neck mass is a thyroid
Thyroid enlargement
Bilobed, anterior and lateral to the trachea.
Cervical teratoma
Complex solid-cystic mass, often displacing the airway.
Lymphatic malformation
Multiloculated cystic lesion crossing tissue planes.
Thyroglossal duct cyst
Small midline cystic lesion along the duct tract.
Foregut duplication cyst
Unilocular cyst near the airway or esophagus.
Vascular lesion
Internal flow and different anatomic distribution.
Location, composition, vascularity, and airway relationship establish the differential.
Reporting
The report should answer the next clinical question
IndicationGraves / TRAb / ATD / fetal finding
BiometryDimensions, circumference or area, percentile source
MorphologySymmetry, echogenicity, focal lesion
VascularityCentral, peripheral, diffuse, reduced, indeterminate
MechanicalAirway relationship, neck position, swallowing, fluid
SystemicFHR, growth, cardiac function, hydrops, bone maturation
ImpressionHyper favored / hypo favored / indeterminate
State the exposure
Interpretation changes when the mother has elevated TRAb/TSI versus ATD exposure.
Name uncertainty
“Indeterminate fetal thyroid function” is safer than overcalling one Doppler feature.
Make escalation explicit
Recommend MFM/endocrinology review when findings may change treatment.
Management
Suspected fetal hyperthyroidism is treated through the mother
1
Classify
Maternal TRAb/TSI + fetal phenotype
2
Treat
Maternal antithyroid drug crosses the placenta
3
Reassess
Maternal labs + serial fetal thyroid/system response
Do not adjust treatment from ultrasound in isolation.
Coordinate Maternal-Fetal Medicine and endocrinology; involve neonatology when late antibodies or therapy create neonatal risk.
Conditional · low evidence
Therapy exception
Block-and-replace has one narrow fetal-treatment exception
Maternal hypothyroidism after definitive Graves treatment
Continue LT4
maintain maternal euthyroidism
+
Add ATD
treat fetal hyperthyroidism
×
Not routine pregnancy therapy
Outside this fetal-treatment scenario, block-and-replace is not used in pregnancy.
Guideline-backed exception
Fetal hypothyroid goiter
Start by removing suppression
1
Correct exposure
Reassess and reduce maternal ATD or excess iodine when clinically safe.
2
Reassess the fetus
Follow gland size, fluid, swallowing, airway, maturation, and the systemic phenotype.
3
Escalate selected cases
Severe persistent compressive goiter may prompt fetal-therapy consultation.
Limited evidence
Intra-amniotic levothyroxine is not routine guidance. Published practice is based largely on observational reports and case series, with variable regimens.
Diagnostic uncertainty
Invasive testing is reserved for consequential uncertainty
Before the gate
Maternal history and therapy
TRAb/TSI
Serial ultrasound phenotype
Multidisciplinary review
Will the result change treatment?
Cordocentesis
Most direct fetal thyroid assessment; approximately 1–2% fetal death risk cited by ATA.
Amniotic fluid
Lower procedural risk, but a normal result does not exclude fetal dysfunction.
Consider at 20–24 weeks only when goiter is present, status remains unclear, and the answer will alter management.
Selected use
Peripartum planning
Delivery planning follows airway physiology, not goiter size alone
1
Define mechanics
Tracheal displacement/compression, neck extension, swallowing, polyhydramnios.
2
Refine imaging
Targeted ultrasound; add fetal MRI when airway anatomy remains uncertain.
3
Build the team
MFM, neonatology, anesthesia, pediatric ENT/surgery, fetal imaging.
4
Choose delivery plan
Standard delivery, prepared neonatal airway support, or selected EXIT strategy.
A large goiter does not automatically mandate cesarean delivery or EXIT.
Base the plan on predicted obstruction, fetal status, obstetric factors, and local expertise.
Expert / observational evidence
Clinic algorithm
A single algorithm aligns the whole team
Current or prior Graves
First-trimester TSH, FT4, TRAb/TSI
ATD use
Any continued pregnancy exposure
Stable Hashimoto + LT4
No risk modifier
→
TRAb/TSI >3× ULN or ATD
Monthly fetal thyroid ultrasound from 18–20 weeks
Concerning fetal phenotype
Targeted thyroid + systems evaluation now
No exposure / no signal
Routine anatomy pathway
→
Classify
Hyper favored · hypo favored · indeterminate
Coordinate
MFM + endocrinology + neonatology as indicated
Treat and plan
Serial response and airway-aware delivery plan
Case resolution
The case changes because the exposure was recognized
First trimester
History triggers testing
Prior Graves treated with radioactive iodine. Maternal hypothyroidism does not remove fetal risk.
Laboratory gate
TRAb/TSI >3× ULN
The fetus enters surveillance even though the mother is euthyroid on levothyroxine.
18–20 weeks onward
Monthly targeted ultrasound
Thyroid, heart rate, growth, fluid, cardiac function, and maturation.
Late pregnancy
Neonatal plan
Repeat antibodies and coordinate newborn thyroid monitoring when risk persists.
The maternal label was misleading. The antibody exposure selected the fetus for surveillance before a late phenotype appeared.
Take-home messages
Clinical pearls
Evidence & controversies
Strong triage guidance. Limited fetal-therapy evidence.
What is known
- TRAb/TSI and ATD exposure identify risk.
- >3× ULN triggers surveillance.
- Goiter does not define function.
- Stable Hashimoto disease does not justify serial thyroid imaging.
What remains uncertain
- Best nomogram across populations.
- Accuracy of Doppler phenotype scores.
- Reduced scan cadence on low-dose ATD.
- Objective airway/EXIT thresholds.
Research priorities
- Prospective fetal thyroid imaging standards.
- Validated multimodal diagnostic models.
- Standardized intra-amniotic LT4 regimens.
- Long-term neurodevelopmental outcomes.
Guideline transparency: No current standalone SMFM Consult Series specifically devoted to fetal thyroid surveillance was identified. This presentation uses ACOG guidance, the current multidisciplinary ATA guideline, AIUM standards, and primary MFM imaging literature.
References
Evidence base
Korevaar TIM, Leung AM, Alexander EK, et al. American Thyroid Association 2026 Guidelines for Thyroid Disease in Preconception, Pregnancy, and Postpartum. Thyroid. 2026;36(5):481–544. doi:10.1177/10507256261445624
American College of Obstetricians and Gynecologists. Thyroid Disease in Pregnancy: Practice Bulletin No. 223. Obstet Gynecol. 2020;135(6):e261–e274. Reaffirmed 2023. ACOG guidance
American College of Obstetricians and Gynecologists. Indications for Outpatient Antenatal Fetal Surveillance: Committee Opinion No. 828. Obstet Gynecol. 2021;137(6):e177–e197. ACOG guidance
Alexander EK, Pearce EN, Brent GA, et al. 2017 Guidelines of the American Thyroid Association for the Diagnosis and Management of Thyroid Disease During Pregnancy and the Postpartum. Thyroid. 2017;27(3):315–389. doi:10.1089/thy.2016.0457
Ranzini AC, Ananth CV, Smulian JC, et al. Ultrasonography of the fetal thyroid: nomograms based on biparietal diameter and gestational age. J Ultrasound Med. 2001;20(6):613–617. doi:10.7863/jum.2001.20.6.613
Huel C, Guibourdenche J, Vuillard E, et al. Use of ultrasound to distinguish between fetal hyperthyroidism and hypothyroidism on discovery of a goiter. Ultrasound Obstet Gynecol. 2009;33(4):412–420. doi:10.1002/uog.6315
American Institute of Ultrasound in Medicine. AIUM Practice Parameter for the Performance of Detailed Second- and Third-Trimester Diagnostic Obstetric Ultrasound Examinations. J Ultrasound Med. 2019;38(12):3093–3100. doi:10.1002/jum.15163
Iijima S. Current knowledge about the in utero and peripartum management of fetal goiter associated with maternal Graves’ disease. Eur J Pediatr. 2019;178:1751–1764. Open-access article
Courtier J, Poder L, Wang ZJ, Westphalen AC, Yeh BM, Coakley FV. Fetal tracheolaryngeal airway obstruction: prenatal evaluation by sonography and MRI. Pediatr Radiol. 2010;40(11):1800–1805. Open-access article
Educational notice. This presentation supports professional education and does not replace individualized clinical judgment, institutional policy, or multidisciplinary consultation. Evidence reviewed August 24, 2026.
OpenMFM.orgOpen-source clinical educationLast evidence review: Aug 24, 2026