1. Purpose
Thyroid hormone is essential for fetal and neonatal growth, brain development, metabolism, and cardiorespiratory stability.
This chapter covers:
- Hypothyroxinemia of prematurity
- Congenital hypothyroidism (primary and central)
- Iodine contrast–associated thyroid dysfunction
- Thyroid disease in infants of mothers with thyroid disorders
- Congenital hyperthyroidism (neonatal Graves' disease)
2. Hypothyroxinemia of Prematurity
Hypothyroxinemia of prematurity is common in infants <32 weeks' gestation. Typical pattern: low T4 with normal or low TSH.
Causes of low thyroid hormone in preterm infants:
- Immature thyroid hormone synthesis
- Immature hypothalamic-pituitary-thyroid (HPT) axis
- Low thyroid-binding globulin
- Glucocorticoids and dopamine suppress TSH secretion
Natural history
Most preterm infants have transient hypothyroxinemia for ~6–8 weeks, with more severe disease at lower gestational ages. It resolves completely in most cases.
3. Why It Matters
| Issue | Practical Meaning |
|---|
| Common in preterm infants | Low T4 with normal TSH may be physiologic and transient in very preterm infants |
| Must distinguish from true hypothyroidism | Primary and central hypothyroidism require different evaluation and treatment decisions |
| Treatment benefit uncertain | Evidence does not clearly show that treating transient hypothyroxinemia improves mortality, neurodevelopment, or RDS severity |
| Usually resolves | In most infants, hypothyroxinemia resolves completely within 4–8 weeks |
4. Evaluation of Low T4 with Normal TSH in a Preterm Infant
Low T4 / Normal TSH in Asymptomatic ELBW or Preterm
STEP 1 — Confirm second newborn screen has been sent (or send it now)
↓
STEP 2 — Send
hospital laboratory free T4 and TSH at the same time
↓
STEP 3 — If thyroid function tests or repeat newborn screen remain abnormal:
→ Send
free T4 by equilibrium dialysis
→ Consult endocrinology when result returns
Free T4 measurement interference
Heparin, furosemide, and high free fatty acids can displace T4 from binding proteins and falsely elevate free T4. Use equilibrium dialysis to get a reliable free T4 in these infants.
5. Findings Suggesting Central Hypothyroidism
Central hypothyroidism = hypothalamic or pituitary dysfunction → low TSH + low free T4. Look for associated features:
- Micropenis
- Cleft lip or cleft palate
- Midline facial hypoplasia
- Nystagmus
- Hypoglycemia
- Prolonged indirect hyperbilirubinemia
- Evidence of ACTH, GH, prolactin, or gonadotropin deficiency
- Central diabetes insipidus
- Structural hypothalamic, pituitary, or brain abnormalities on imaging
Central hypothyroidism = evaluate all pituitary axes
Do not treat central hypothyroidism with levothyroxine in isolation before ruling out ACTH deficiency — thyroid replacement can precipitate adrenal crisis in undiagnosed combined pituitary deficiency.
6. Congenital Hypothyroidism
- Inadequate thyroid hormone production present from birth
- Affects ~1 in 3,000 infants — one of the most common neonatal endocrine disorders
- Untreated → irreversible neurocognitive injury (cretinism)
- Most newborns with congenital hypothyroidism have no obvious clinical signs at birth — newborn screening is essential
7. Etiology of Congenital Hypothyroidism
| Type | Mechanism |
|---|
| Primary congenital hypothyroidism | Thyroid gland dysgenesis or dyshormonogenesis → low thyroid hormone + high TSH |
| Thyroid dysgenesis | Agenesis, ectopia, or hypoplasia — most common cause of primary congenital hypothyroidism |
| Central congenital hypothyroidism | Hypothalamic or pituitary dysfunction → low TSH + low thyroid hormone |
| Peripheral defects | Disorders of thyroid hormone transport, metabolism, or action — rare |
| Iodine deficiency | Most common cause worldwide, especially in low-resource settings |
8. Clinical Findings Suggesting Congenital Hypothyroidism
Many infants are asymptomatic at birth. Signs may emerge over days to weeks if untreated:
- Goiter
- Poor linear growth
- Prolonged indirect hyperbilirubinemia
- Wide posterior fontanelle
- Lethargy and hypotonia
- Feeding difficulty and constipation
- Hypothermia
- Dry skin, coarse or puffy facies, and hoarse cry
Why screening finds it before symptoms
Clinical features of congenital hypothyroidism develop gradually and may not be apparent until weeks of age. Newborn screening identifies cases before signs appear, allowing treatment before brain injury occurs.
9. Confirming an Abnormal Newborn Screen
Abnormal newborn screen results are not diagnostic and require confirmatory venipuncture free T4 and TSH.
| Lab Pattern | Interpretation | Action |
|---|
| High TSH + Low free T4 |
Primary hypothyroidism |
Start levothyroxine promptly |
| High TSH + Normal free T4 |
Subclinical hypothyroidism |
Treat if TSH >20 mIU/L; monitor and repeat for milder elevation |
| Low/Normal TSH + Low free T4 |
Central hypothyroidism, nonthyroidal illness, prematurity, or acute illness |
Repeat testing; evaluate for other pituitary hormone deficiencies |
Consult endocrinology for all concerning patterns
Pediatric endocrinology should be contacted for treatment decisions, ongoing management, and follow-up in all cases with abnormal confirmatory thyroid function.
10. Treatment of Congenital Hypothyroidism
- Levothyroxine is the preferred treatment
- Usual starting dose: 10–15 mcg/kg/day — individualize based on severity
- Goal: rapid correction of hypothyroxinemia while maintaining euthyroidism with close monitoring
Levothyroxine Administration
Starting dose: 10–15 mcg/kg/day PO
Tablets preferred over liquid (more consistent dosing)
Crush tablet, mix with small amount of water or breast milk
Give at SAME TIME daily
AVOID: soy formula, calcium, iron products (impair absorption)
11. Prognosis of Congenital Hypothyroidism
- With early and adequate treatment: neurodevelopmental outcomes are generally excellent, normal growth and intelligence usually preserved
- Poorer outcomes associated with: delayed diagnosis, more severe disease, or inadequate thyroid hormone replacement
- Emphasizes the critical importance of newborn screening and prompt treatment
12. Iodinated Contrast and Thyroid Dysfunction
- Term and preterm infants (especially those with congenital heart disease) may develop temporary hypothyroidism after iodinated contrast exposure
- Monitoring: begin 2–4 weeks after iodinated contrast exposure and continue until thyroid tests normalize
Don't forget contrast-related hypothyroidism
Cardiac catheterization and imaging procedures using iodinated contrast are common in NICU infants with CHD. Always flag these patients for post-procedure thyroid monitoring at 2–4 weeks.
13. Infant Born to a Mother with Thyroid Disease
First step: determine the mother's thyroid diagnosis. Autoimmune thyroid disease is the most common maternal thyroid disorder.
Key maternal antibodies
- TPOAb (thyroid peroxidase antibody) — marker of autoimmune disease
- TgAb (thyroglobulin antibody) — marker of autoimmune disease
- TRAb (TSH receptor antibody) — can stimulate OR block the TSH receptor
TRAb effect on the neonate depends on whether the antibody is stimulating (→ neonatal hyperthyroidism) or blocking (→ neonatal hypothyroidism), and whether the mother is also on antithyroid medication.
Autoimmune hyperthyroidism carries greater fetal and neonatal risk than autoimmune hypothyroidism.
14. Maternal Graves' Disease
- Caused by stimulating TRAb that cross the placenta → fetal or neonatal hyperthyroidism
- Maternal antithyroid medications (PTU, methimazole) also cross the placenta → may cause short-term neonatal hypothyroidism
- A mother with prior thyroidectomy or radioiodine may still have circulating TRAb that causes neonatal thyroid disease — past history is critical
Prior thyroidectomy ≠ safe
A mother who is now clinically hypothyroid after definitive treatment for Graves' disease may still have circulating TRAb. Check neonatal thyroid tests and TRAb/TSI regardless of the mother's current thyroid status.
15. Maternal Hashimoto Disease or Acquired Hypothyroidism
- Maternal autoimmune hypothyroidism: usually low risk for clinically significant neonatal thyroid dysfunction
- Routine newborn screening is usually sufficient when the mother has autoimmune hypothyroidism and the infant is asymptomatic
- Non-autoimmune hypothyroidism (e.g., thyroid cancer–related): generally not increased neonatal risk; routine screening adequate
- Hereditary or familial congenital hypothyroidism: neonatal risk is increased — genetics or endocrinology consultation may help guide evaluation
16. Unknown Maternal Thyroid Diagnosis
- If maternal diagnosis is unclear, the highest-risk scenario is current or prior autoimmune hyperthyroidism
- If autoimmune hyperthyroidism can be ruled out and the infant is asymptomatic: close follow-up of newborn screening results is adequate
- Any symptomatic infant: send serum TSH, free T4, total T3, and TRAb urgently with endocrinology consultation
17. Congenital Hyperthyroidism
- Rare but may significantly affect prenatal and postnatal growth and development
- Most cases occur in the setting of maternal Graves' disease
- Only ~1–5% of infants born to women with Graves' disease develop neonatal hyperthyroidism
- Overt neonatal hyperthyroidism: estimated ~1 in 50,000 newborns
18. Pathogenesis of Congenital Hyperthyroidism
- Most neonatal Graves': transplacental transfer of IgG-type stimulating TRAb → fetal thyroid stimulation → elevated thyroid hormone
- Maternal antibodies may persist after thyroidectomy, radioiodine, or in some Hashimoto-related states
- Neonatal Graves' usually remits by 4–5 months as maternal antibodies clear
- Persistent disease into childhood → suggests activating TSH receptor mutations (not transient antibody-mediated)
19. Prenatal Signs of Congenital Hyperthyroidism
- Fetal tachycardia
- Cardiac arrhythmia
- Fetal growth restriction
- Advanced bone age
- Goiter
- Craniosynostosis or microcephaly
- Prematurity or fetal demise
20. Postnatal Signs of Congenital Hyperthyroidism
| System | Signs |
|---|
| Neurological | Irritability, hyperexcitability, sleep disturbance |
| Cardiovascular | Tachycardia, hypertension, cardiac failure |
| Constitutional | Fever, flushing, sweating |
| Respiratory | Tachypnea, respiratory distress, pulmonary hypertension |
| Neck/Eyes | Goiter, eyelid retraction, stare |
| Nutrition | Feeding difficulty or poor weight gain despite increased appetite |
| GI | Diarrhea or vomiting |
| Other | Petechiae, hyperbilirubinemia, jaundice, hepatosplenomegaly, bilateral gynecomastia |
| Head | Small anterior fontanelle, craniosynostosis, or microcephaly |
21. Timing of Neonatal Graves' Presentation
| Maternal Situation | Expected Neonatal Timeline |
|---|
| Mother NOT on antithyroid medication |
Infant may be hyperthyroid at birth |
| Mother ON antithyroid medication |
Infant may be euthyroid or hypothyroid at birth, then become hyperthyroid at 7–10 days as maternal medication clears |
Do not discharge before day 10 without re-checking
Infants of mothers on antithyroid medication may appear normal at birth then develop hyperthyroidism at 7–10 days. Schedule follow-up or keep monitoring through that window.
22. Diagnosis of Congenital Hyperthyroidism
- Confirmed by: high T4 or free T4 + high T3 + suppressed TSH
- Interpret using gestational-age appropriate reference values
- For infants of mothers with Graves': send TRAb and TSI from cord blood or as soon as possible after birth
23. Treatment of Congenital Hyperthyroidism
- Start treatment promptly once diagnosis is confirmed
- Methimazole is the antithyroid medication of choice
- PTU is not recommended — more frequent and severe adverse effects, including hepatotoxicity
- Beta-blockers as adjunctive therapy for neuromuscular and cardiovascular hyperactivity
- Iodine may be added if hyperthyroidism is not controlled with methimazole + beta-blocker
- Glucocorticoids may be used in extremely ill infants
- Average treatment duration: 2–3 months (range 1–6 months)
Why PTU is avoided in neonates
PTU carries a risk of hepatotoxicity and agranulocytosis that is higher than methimazole. The FDA has issued a black-box warning against PTU in pediatric patients except when methimazole is not tolerated.
24. Breastfeeding and Maternal Antithyroid Drugs
- Lactating mothers on antithyroid therapy can continue breastfeeding within these dose limits:
- PTU: ≤400 mg/day
- Methimazole: ≤40 mg/day
25. Practical Bedside Algorithm
Thyroid Disorders — Bedside Approach
STEP 1 — Review newborn screen AND maternal thyroid history
↓
STEP 2 — Abnormal newborn screen → confirm with venipuncture free T4 and TSH
↓
STEP 3a — High TSH + low free T4 →
start levothyroxine promptly
STEP 3b — Low/normal TSH + low free T4 → consider central hypothyroidism, prematurity, illness → evaluate pituitary axes
↓
STEP 4 — Maternal Graves' (current or past): send neonatal TSH, free T4, T3,
TRAb/TSI early
↓
STEP 5 — Suspected neonatal hyperthyroidism → TSH, free T4, total T3, TRAb/TSI + urgent endocrinology
↓
STEP 6 —
Iodinated contrast given? Check thyroid function starting 2–4 weeks after exposure
26. Common Mistakes
| Mistake | Better Action |
|---|
| Treating all low T4 in preterm infants as congenital hypothyroidism | Distinguish transient hypothyroxinemia from primary and central hypothyroidism — not all low T4 requires treatment |
| Ignoring central hypothyroidism clues | Micropenis, hypoglycemia, midline defects, and prolonged jaundice should trigger pituitary evaluation |
| Assuming newborn screen alone is diagnostic | Confirm all abnormal results with venipuncture free T4 and TSH |
| Giving levothyroxine with soy, iron, or calcium | These impair levothyroxine absorption — give separately |
| Missing neonatal Graves' in a mother who is now hypothyroid | TRAb can persist after thyroidectomy or radioiodine treatment and still affect the neonate |
| Checking only at birth in infants of mothers on antithyroid drugs | Hyperthyroidism may appear at 7–10 days as maternal medication clears — re-check then |
| Forgetting contrast-related hypothyroidism | Monitor thyroid function 2–4 weeks after iodinated contrast exposure |
27. Parent Explanation
What to Tell Families
"The thyroid gland makes hormones essential for the baby's growth, metabolism, and especially brain development."
"If the newborn screen is abnormal, we confirm it with a blood test — the screen is not the final diagnosis."
"If treatment is needed, levothyroxine is a safe and effective way to replace the missing thyroid hormone, and outcomes are excellent when started early."
"If you had Graves' disease — even if you had surgery or radioiodine treatment in the past — your baby may need special thyroid monitoring because your antibodies can cross the placenta and temporarily affect the baby's thyroid."
Key Takeaways — Chapter 4.2
- Low T4 + normal TSH is common in very preterm infants (<32 wks) — usually transient (resolves 6–8 wks), treatment benefit unclear
- Free T4 by equilibrium dialysis is most reliable in sick preterm infants (heparin/furosemide falsely elevate standard free T4)
- Congenital hypothyroidism: ~1 in 3,000; may be clinically silent at birth; untreated → irreversible neurocognitive injury
- High TSH + low free T4 → primary hypothyroidism → start levothyroxine promptly
- Low/normal TSH + low free T4 → central hypothyroidism, prematurity, or illness → evaluate all pituitary axes before treating
- Levothyroxine: 10–15 mcg/kg/day; tablets preferred over liquid; avoid soy, calcium, iron (impair absorption)
- Iodinated contrast → monitor thyroid function starting 2–4 weeks post-exposure
- Maternal Graves' (current or past) → send TRAb/TSI + neonatal thyroid labs early
- Mother on antithyroid medication → infant may become hyperthyroid at 7–10 days as medication clears
- Prior thyroidectomy/radioiodine ≠ safe for neonate — TRAb can persist
- Neonatal hyperthyroidism treatment: methimazole (preferred) + beta-blocker; avoid PTU (hepatotoxicity)
- Neonatal Graves' typically remits by 4–5 months as maternal antibodies clear
References
- Baylor College of Medicine — Guidelines for Acute Care of the Neonate, current edition: Chapter 4.2
- Leger J, et al. Congenital hypothyroidism. Best Pract Res Clin Endocrinol Metab. 2014
- American Academy of Pediatrics, Rose SR, et al. Update of newborn screening and therapy for congenital hypothyroidism. Pediatrics. 2006
- van Trotsenburg ASP, et al. Congenital hypothyroidism: a 2021–2022 consensus guidelines update. Eur Thyroid J. 2023
- Polak M, et al. Fetal and neonatal thyroid function. Endocr Dev. 2014
- Smith J, et al. Hypothyroxinemia in preterm infants. Curr Opin Pediatr. 2000
- van der Kaay DC, et al. Management of neonates born to mothers with Graves' disease. Pediatrics. 2016
- Alexander EK, et al. 2017 Guidelines for the diagnosis and management of thyroid disease during pregnancy and the postpartum. Thyroid. 2017