Thyroid in Neonates

1. Fetal Thyroid Development

2. Normal Neonatal Thyroid Physiology

• TSH surges (neonatal TSH surge) within minutes of delivery, driven by cold exposure and TRH
• This triggers a rise in T4 conversion to T3
• T4 levels transiently rise (transient hyperthyroxinemia may reflect thermogenesis adaptation)
• TSH normalizes to adult levels within a few days via T3/T4 negative feedback
• T4 and T3 return to normal adult values within 4–6 weeks
• The hypothalamic-pituitary-thyroid (HPT) axis fully matures at 1–2 months after birth

• HPT axis is immature; the physiological TSH surge is dramatically lower or absent
• FT4 and FT3 are low with paradoxically normal TSH
• Takes 3–8 weeks to reach levels similar to term infants
• Transient hypothalamic hypothyroidism of prematurity occurs in up to 50% of infants born <28 weeks
• Dopamine (used in NICU) can suppress TSH release further

3. Congenital Hypothyroidism (CH)

Incidence

• Estimated 1 in 2,000–4,000 newborns (higher with sensitive TSH cut-offs)
• Higher incidence in Asians, Hispanics, premature infants, and older mothers
• Prior to newborn screening, estimated at 1 in 7,000

Classification

• Thyroid dysgenesis (85%): agenesis, ectopy, or hypoplasia
• Dyshormonogenesis (15%): structurally normal gland with hormone synthesis defect

• Endemic iodine deficiency (most common worldwide)
• Maternal antithyroid drug exposure
• Transfer of maternal TSH-receptor blocking antibodies
• Maternal iodine excess (e.g., amiodarone, contrast media)
• Liver hemangiomas (excess deiodinase 3 activity)
• Certain genetic defects

Causes by Category (Harrison's, 2025)

• Thyroid dysgenesis: 65%
• Dyshormonogenesis (inborn errors of TH synthesis): 30%
• TSH-receptor antibody mediated: 5%
• Developmental abnormalities twice as common in girls

4. Genetics of Congenital Hypothyroidism

5. Clinical Features

• Lethargy, increased sleep
• Prolonged neonatal jaundice
• Myxedematous facies
• Large anterior fontanelle
• Macroglossia
• Distended abdomen, umbilical hernia
• Hypothermia, hypotonia

• Poor sucking and feeding difficulties
• Constipation
• Developmental delay
• Cognitive and growth retardation
• Myxedema
• Decreased activity

6. Neonatal Screening

• TSH first (most common) → reflex T4 if TSH elevated: misses central hypothyroidism
• T4 first → reflex TSH if T4 low
• Combined TSH + FT4 ± Tg: differentiates primary vs. central causes

• False-positive TSH elevations in the first 2 days of life (physiologic TSH surge)
• Transplacental maternal TSH heterophile antibodies → false-positive TSH
• Premature/acutely ill infants: late TSH rise → may be missed on first sample → many programs do a second screen
• Dopamine suppresses TSH in NICU infants
• Seasonal variation: false-positive rate higher in winter (0.9%) vs. summer (0.6%)

7. Treatment

• Levothyroxine (LT4) is the treatment of choice
• Goal: normalize serum TSH and raise T4 to age-appropriate levels
• If started within the first 2 weeks of life → prevents mental retardation in >90% of children
• Dose is weight-based and adjusted as the infant grows

8. Neonatal Graves Disease

• Fetal/neonatal hyperthyroidism
• Thyrotoxicosis, tachycardia, goiter, advanced bone age

• Creasy & Resnik's Maternal-Fetal Medicine, Ch. 61
• Tietz Textbook of Laboratory Medicine, 7th Ed., Ch. 26
• Harrison's Principles of Internal Medicine, 22nd Ed. (2025), Ch. 395

Meromelia in Neonates — Causes

Definition

• Hemimelia — absence of a specific bone segment (e.g., absent fibula)
• Phocomelia — hands/feet attached directly to the trunk by small, irregularly shaped bones ("seal limbs")
• Micromelia — all segments present but abnormally short (sometimes classified separately)

Critical Period

Causes

1. Teratogenic (Drug/Chemical) Causes

Thalidomide is the classic cause. The sensitive period was 20–36 days after fertilization. Currently still used for leprosy, multiple myeloma, and autoimmune diseases — absolutely contraindicated in women of childbearing age. Teratogen-induced meromelia is usually bilateral and symmetric.

2. Genetic / Chromosomal Causes

• Trisomy 18 (Edwards syndrome) — associated with various limb malformations
• Other chromosomal aneuploidies

• Mutations in HOX genes (HOXA/HOXD family) — master regulators of limb patterning along the proximal-distal and anterior-posterior axes
• BMP (bone morphogenetic proteins) pathway mutations
• SHH (sonic hedgehog) mutations — anterior-posterior patterning
• WNT7 mutations — dorsal-ventral limb patterning
• EN1 mutations
• TBX5 mutations (chromosome 12q24.1) → Holt-Oram syndrome (upper limb abnormalities + congenital heart defects)
• HOXA13 mutations → hand-foot-genital syndrome
• HOXD13 mutations → synpolydactyly

3. Vascular Disruption / Ischemic Causes

4. Amniotic Band Syndrome (ABS)

• Occurs in 1:1,200 to 1:15,000 live births
• Fibrous bands of amnion encircle and constrict fetal limbs
• Second or third trimester disruption → constriction rings, lymphedema, and limb amputation (producing transverse meromelia)
• First trimester disruption → craniofacial and visceral defects
• Most commonly affects hands/feet (90%), umbilical cord (30%), abdomen (20%)
• Results in: limb/digit amputation, constriction rings, acrosyndactyly (distal digit fusion)
• ABS-related defects are typically asymmetric and random (unlike teratogen-induced defects)

5. Mechanical / Intrauterine Factors

• Oligohydramnios — reduced amniotic fluid commonly associated with limb deformations (compression effects)
• Intrauterine constraint and mechanical pressure during the critical developmental window

6. Multifactorial / Unknown

• A combination of genetic predisposition and environmental triggers
• Most cases of thyroid dysgenesis and limb defects occurring sporadically may have polygenic/epigenetic components
• Many cases remain idiopathic despite full workup

Summary Table

• The Developing Human: Clinically Oriented Embryology — Ch. 16 (Birth Defects of Limbs), Ch. 20 (Teratology)
• Langman's Medical Embryology — Ch. 12 (Limb Defects)
• Creasy & Resnik's Maternal-Fetal Medicine — Ch. 26 (Amniotic Band Syndrome)

Neonate Born to a Mother with Sickle Cell Anemia — Workup

Context: Why the Neonate Needs a Workup

1. Inheriting SCD or sickle cell trait (depending on the father's hemoglobin genotype)
2. Neonatal complications related to maternal SCD during pregnancy (growth restriction, prematurity, anemia)
3. Early-onset complications of SCD itself if the infant is affected

The key principle: SCD is not clinically apparent at birth because fetal hemoglobin (HbF, α2γ2) predominates and does not sickle. Symptoms emerge only as HbF is replaced by HbS over the first 6–12 months of life. This is exactly why newborn screening is critical — it identifies affected infants before symptoms begin.

Step 1 — Newborn Screening (Universal + Targeted)

Hemoglobin Electrophoresis (Gold Standard)

• The initial and definitive diagnostic test for SCD in the neonatal period
• Performed via heel-stick dried blood spot (filter paper card) or cord blood
• Identifies the specific hemoglobin pattern present

Important pitfall: The Sickledex (solubility test) and sickle preparation tests can give false-negative results in neonates because the high percentage of HbF dilutes HbS below the detection threshold. These tests should not be used for neonatal screening. Hemoglobin electrophoresis or HPLC (high-performance liquid chromatography) is required.

• In the US, newborn screening for SCD is universal and mandatory in all 50 states
• Screening is ideally done at 2–4 days of life for term infants

HPLC (High-Performance Liquid Chromatography)

• Increasingly used alongside or instead of electrophoresis in newborn screening programs
• Quantifies each hemoglobin fraction precisely and identifies variants

Confirmatory Testing

• Any positive screen must be confirmed with a second independent sample (serum or repeat filter card) using a different method (e.g., electrophoresis at alkaline + acid pH, or HPLC + IEF)
• Confirmation should occur by 4–8 weeks of age

Step 2 — Parental Genotype Assessment

• Father's hemoglobin status must be determined to assess the neonate's risk
• Father's genotype determines whether the baby can have:
  • HbSS (if father carries HbS)
  • HbSC (if father carries HbC)
  • HbSβ-thalassemia (if father carries β-thal mutation)
  • HbAS trait only (if father is HbAA or HbAS)

Step 3 — Baseline Laboratory Workup (if infant is screen-positive)

In sickle cell disease, typical reticulocyte count is 3–4× the upper limit of normal due to compensated hemolysis. A reticulocyte count ≤3% of normal may signal aplastic crisis; >12% with nucleated RBCs suggests rapid hemolysis.

Step 4 — Imaging and Additional Studies

• Transcranial Doppler (TCD) ultrasound: Recommended starting at 2 years of age in HbSS and HbSβ0-thalassemia patients to screen for stroke risk (velocity >200 cm/sec indicates high risk requiring transfusion)
• Echocardiogram: Not routine at birth, but relevant if cardiac symptoms emerge
• Ophthalmology: Screening for retinal disease starts in older children

Step 5 — Preventive Interventions (Initiated in Infancy)

Penicillin Prophylaxis (Critical)

• Start: As soon as diagnosis is confirmed — by 2 months of age
• Dose (Penicillin V potassium):
  • Age 2 months to <3 years: 125 mg PO BID
  • Age 3–5 years: 250 mg PO BID
  • Can be discontinued after age 5 if the child received all recommended pneumococcal vaccinations and has had no invasive pneumococcal infection
• Covers encapsulated organisms (especially S. pneumoniae)

Immunizations

• Pneumococcal vaccines: PCV15/PCV20 (primary series) + PPSV23 after age 2
• Meningococcal vaccines
• Haemophilus influenzae type b (Hib) vaccine
• Annual influenza vaccine

Folic Acid Supplementation

• Ongoing hemolysis increases folate demand; supplementation is standard

Step 6 — Hydroxyurea Considerations

• Hydroxyurea (HU) is now recommended for all children with HbSS or HbSβ0-thalassemia starting at 9 months of age (regardless of clinical severity), per NHLBI guidelines
• Mechanism: induces HbF production → reduces sickling, hemolysis, pain crises, ACS, and mortality
• Not typically initiated in the neonatal period but should be planned from diagnosis

Summary Workup Table

• The Harriet Lane Handbook, 23rd Edition — Ch. 14 (Hematology); Ch. Formulary (Penicillin V)
• Rosen's Emergency Medicine, Ch. 109 (Sickle Cell Disease)
• Henry's Clinical Diagnosis and Management by Laboratory Methods — Ch. 38
• Creasy & Resnik's Maternal-Fetal Medicine, Ch. 55
• Mosca A, et al. "Screening for sickle cell disease: focus on newborn investigations." Clin Chem Lab Med 2024 [PMID: 38888156]