Congenital Hypothyroidism: Natural History, Overview, and Treatment
Natural History and Overview
Congenital hypothyroidism (CH) is a treatable endocrine disorder that, if left untreated, leads to irreversible intellectual disability and growth impairment, but with early detection through newborn screening and prompt levothyroxine treatment, neurodevelopmental outcomes are grossly normal. 1, 2
Clinical Presentation Without Treatment
The natural history of untreated CH is devastating. Clinical manifestations develop gradually and include:
- Early signs: Sleepiness, failure to wake for feeds, poor and slow feeding, cold extremities, prolonged neonatal jaundice (beyond 2 weeks), lethargy, and hypotonia 1
- Physical features: Macroglossia, umbilical hernia, dry skin, coarse/puffy face, large posterior fontanelle, large anterior fontanelle, and wide sagittal suture 1
- Skeletal markers: Absence of femoral and tibial epiphyses on knee x-ray indicates severe intrauterine hypothyroidism and correlates with worse IQ outcomes 1
Severity Classification
CH severity is assessed through multiple parameters 1:
- Severe: Free T4 (FT4) < 5 pmol/L
- Moderate: FT4 5-10 pmol/L
- Mild: FT4 10-15 pmol/L
A critical threshold exists at FT4 < 5.5 pmol/L (equivalent to total T4 < 40 nmol/L), below which a 10-point IQ deficit occurs compared to children with normal T4 levels 1.
Etiology and Permanence
Approximately 40% of CH cases involve normally located thyroid glands (in situ thyroid gland), and among these with unexplained mechanisms, 56% prove to be transient rather than permanent 3. This highlights the heterogeneous nature of CH and the critical importance of re-evaluation.
Screening and Diagnosis
Newborn Screening Thresholds
If capillary TSH from newborn screening is ≥40 mU/L whole blood, start treatment immediately after obtaining a venous sample without waiting for confirmatory results 1. If capillary TSH is <40 mU/L, you may wait for venous results provided they are available within 24 hours 1.
Confirmatory Testing and Treatment Initiation
The baby must be seen for clinical assessment and venous thyroid function testing on the day of referral or the next day at the latest 1.
Treatment initiation criteria 1:
- Start immediately if venous FT4 is below age-specific norms, regardless of TSH level (strongest recommendation)
- Start immediately if venous TSH ≥20 mU/L, even if FT4 is normal
- For TSH 6-20 mU/L with normal FT4 in a well baby beyond 21 days: Perform diagnostic imaging, then either:
- Start levothyroxine immediately and retest off treatment later, OR
- Retest in 2 weeks without treatment (discuss with family)
Special Populations Requiring Second Screening
Second screening is essential for 1:
- Preterm neonates
- Low birth weight (LBW) and very low birth weight (VLBW) infants
- Ill newborns in NICU
- Specimens collected within first 24 hours of life
- Multiple births (particularly same-sex twins)
Diagnostic Imaging
Thyroid imaging should be performed using radioisotope scanning (scintigraphy) with or without perchlorate discharge test, or ultrasonography, or both—but imaging must never delay treatment initiation 1. Combining both modalities improves diagnostic accuracy and helps distinguish thyroid dysgenesis from dyshormonogenesis 1.
Treatment
Initial Levothyroxine Dosing
The recommended initial levothyroxine (L-T4) dose is 10-15 mcg/kg/day, with infants having severe disease (very low pretreatment FT4) receiving the highest initial dose (15 mcg/kg/day), and those with mild-to-moderate hypothyroidism receiving lower doses (10 mcg/kg/day) 1, 2.
Administration Guidelines
L-T4 tablets should be crushed and administered via small spoon, suspended in a few milliliters of water or breast milk 1. Key administration principles 1:
- Administer consistently at the same time each day (morning or evening)
- Can be given before feeding or with food, but maintain consistency
- Avoid concurrent administration with soy, iron, and calcium
- Use caution with vitamin D during first weeks of life
- Brand formulations are preferred over generics, at least in infancy
If oral administration is impossible, intravenous L-T4 can be used at 80% of the oral dose 1.
Treatment Goals
TSH must be maintained in the age-specific reference range, while FT4 should be maintained in the upper half of the age-specific reference range 1. This is critical because rapid normalization of thyroid hormone levels within the first 2 weeks and maintenance of relatively higher FT4 concentrations during the first year lead to better intellectual outcomes 1.
Monitoring and Follow-Up
Testing Schedule
Measure serum FT4 and TSH at least 4 hours after the last L-T4 dose 1. The monitoring schedule is intensive 1:
- First follow-up: 1-2 weeks after starting treatment
- Until TSH normalizes: Every 2 weeks
- Age 0-12 months: Every 1-3 months after TSH normalization
- Age 1-3 years: Every 2-4 months
- After age 3 years until growth completion: Every 3-12 months
- After any dose change: Recheck in 4-6 weeks
More frequent evaluations are required if compliance is questioned or abnormal values are obtained 1.
Common Pitfall to Avoid
Do not reduce L-T4 dose based on a single elevated FT4 concentration during treatment 1. This is a critical error that can lead to undertreatment during the crucial neurodevelopmental window.
Re-evaluation for Permanent vs. Transient CH
When Re-evaluation is Mandatory
Re-evaluation off treatment is mandatory when 1:
- No etiological diagnostic assessment was performed in infancy
- Treatment was started in a preterm or sick infant
- Initial evaluation showed a normally located gland (with or without goiter)
- Positive thyroid antibodies were present in the neonate
- No increase in L-T4 dose has been required since infancy
- No enzyme defect has been identified genetically
When Re-evaluation is NOT Needed
Skip re-evaluation if thyroid dysgenesis (ectopy or athyreosis) was conclusively shown on imaging, or if dyshormonogenesis was confirmed by molecular genetic testing (except DUOX2 mutations or Pendred syndrome) 1.
Timing of Re-evaluation
Re-evaluation should normally occur after age 3 years 1, as myelination of the central nervous system is completed by 36-40 months. However, earlier re-evaluation (from age 1 year) may be considered if:
- TSH receptor or thyroid peroxidase antibodies were detected in the newborn
- A eutopic, normally sized gland was found on ultrasound 1
Phase out L-T4 over 4 weeks before retesting to allow accurate assessment of thyroid function 1.
Associated Conditions
All neonates with high TSH must be examined carefully for congenital malformations, particularly cardiac defects (septal defects), renal abnormalities, and dysmorphic features 1. The prevalence of these malformations is significantly higher in CH patients than in the general population 1.
Special Considerations
Pre-existing Cardiac Insufficiency
In children with pre-existing cardiac insufficiency, introduce L-T4 at 50% of the target dose 1. This prevents potential cardiac complications from rapid thyroid hormone replacement.
Family Education
Parents must receive written instructions on L-T4 treatment to ensure compliance 1. The clinician or pharmacist should demonstrate how to give the first dose 1.
School-Age Considerations
When the child reaches nursery or school age, educators and teachers should NOT be informed about the CH diagnosis to avoid stigmatization and labeling 1.