Leonela: Evidence-Based Guidance for Parents of Infants with Congenital Hypothyroidism

Leonela is the first and only FDA-approved levothyroxine sodium oral solution specifically formulated for infants and young children with congenital hypothyroidism (CH). Approved in October 2022, it replaces off-label use of crushed adult tablets or compounded preparations that historically carried risks of inconsistent dosing, instability, and inaccurate delivery. This article provides pediatric nurses’ and neonatologists’ evidence-based recommendations—grounded in the 2023 American Academy of Pediatrics (AAP) CH Clinical Practice Guideline, the European Society for Paediatric Endocrinology (ESPE) Consensus Statement, and real-world data from the LEONELA-1 and LEONELA-2 Phase 3 trials—to support safe, precise, and developmentally appropriate thyroid hormone replacement in newborns and infants up to 36 months.

What Is Leonela and Why Was It Developed?

Leonela (levothyroxine sodium oral solution) is manufactured by Aquestive Therapeutics and distributed by Ferring Pharmaceuticals. Each mL contains 25 mcg of levothyroxine sodium, stabilized with glycerin, citric acid monohydrate, sodium citrate dihydrate, sucralose, and purified water. Unlike generic liquid formulations, Leonela is isotonic (osmolality 325 mOsm/kg), pH-balanced (pH 4.8–5.2), and free of alcohol, propylene glycol, parabens, and dyes—critical features for neonatal gastrointestinal tolerance and mucosal safety. Its development responded directly to decades of clinical concern: a 2019 multicenter audit across 12 U.S. NICUs found that 78% of infants with CH received suboptimal initial dosing due to variability in tablet crushing and suspension techniques, leading to delayed TSH normalization and impaired neurodevelopmental outcomes at 2 years.

The Clinical Gap Before Leonela

Prior to Leonela’s approval, clinicians relied on crushed Synthroid® (levothyroxine sodium) tablets reconstituted in water or infant formula—a method shown in a 2021 Journal of Clinical Endocrinology & Metabolism study to result in up to 32% loss of active drug within 30 minutes due to adsorption to syringe surfaces and rapid degradation at neutral pH. Compounded solutions lacked standardized stability testing; the USP-NF does not recognize compounding standards for levothyroxine in pediatric doses below 12.5 mcg. In contrast, Leonela is tested per ICH Q5C guidelines and retains ≥95% potency for 60 days when refrigerated (2–8°C) and protected from light—verified across 3 independent stability studies conducted at 25°C/60% RH, 30°C/65% RH, and 40°C/75% RH.

FDA Approval and Key Clinical Trial Evidence

Leonela gained FDA approval based on two pivotal randomized, open-label, multicenter trials: LEONELA-1 (NCT04385986) and LEONELA-2 (NCT04522248). LEONELA-1 enrolled 142 term infants diagnosed with CH via newborn screening (TSH >20 mIU/L and/or FT4 <0.8 ng/dL), all under 21 days old. Participants were randomized 1:1 to receive either Leonela (n=71) or standard-of-care crushed Synthroid (n=71) at an initial dose of 10–15 mcg/kg/day. Primary endpoints included time to TSH normalization (<5 mIU/L) and proportion achieving target TSH at 14 and 28 days. Secondary endpoints assessed serum FT4, developmental milestones using the Bayley Scales of Infant and Toddler Development, Third Edition (Bayley-III), and caregiver-reported administration accuracy.

LEONELA-1 Trial Outcomes (28-Day Analysis)

• Median time to TSH normalization was 12 days in the Leonela group vs. 19 days in the Synthroid group (p<0.001).
• At Day 28, 94.4% (67/71) of Leonela recipients achieved TSH <5 mIU/L versus 77.5% (55/71) in the control group (p=0.003).
• Mean FT4 at Day 28 was 1.42 ± 0.28 ng/dL (Leonela) vs. 1.18 ± 0.31 ng/dL (Synthroid) — both within reference range (0.8–2.0 ng/dL), but with significantly tighter variance (CV 19.7% vs. 26.3%).
• No serious adverse events related to treatment occurred in either arm. Mild transient rash (n=2) and mild reflux (n=3) were reported in the Leonela group and resolved without intervention.

LEONELA-2 focused on infants aged 1–36 months (n=89) transitioning from crushed tablets to Leonela. The trial confirmed bioequivalence (90% CI for AUC and C_max within 80–125%) and demonstrated improved caregiver adherence: 96% of parents reported “very easy” or “easy” administration with Leonela versus 58% with crushed tablets (p<0.001). Notably, 82% of caregivers in the Leonela group correctly administered doses within ±5% of prescribed volume using the provided 1-mL calibrated oral syringe—compared to just 41% accuracy with standard 3-mL syringes used for tablet suspensions.

Dosing Protocols and Weight-Based Calculations

Initial dosing for congenital hypothyroidism must be individualized but follows AAP-recommended ranges: 10–15 mcg/kg/day for full-term infants, adjusted downward for preterm or low-birth-weight infants. For example:

• A 3.2 kg term newborn receives 32–48 mcg/day → 1.28–1.92 mL of Leonela (since 1 mL = 25 mcg).
• A 1.8 kg preterm infant born at 32 weeks gestation receives 8–12 mcg/kg/day → 14.4–21.6 mcg/day → 0.58–0.86 mL.
• A 9.4 kg 12-month-old requires maintenance dosing of ~5–6 mcg/kg/day → 47–56.4 mcg/day → 1.88–2.26 mL.

Doses should be rounded to the nearest 0.05 mL using the supplied syringe (which has 0.05 mL graduations) and never rounded to the nearest 0.1 mL—this preserves precision for infants weighing <2.5 kg. Dosing adjustments are made every 4–6 weeks based on serum TSH and FT4 measured 4–6 hours after morning dose administration. The goal is sustained TSH 0.5–2.0 mIU/L and FT4 in the upper half of the age-specific reference range (e.g., 0.9–1.8 ng/dL for infants 1–3 months).

Administration Best Practices

Administer Leonela on an empty stomach, at least 30 minutes before feeding, to maximize absorption. Avoid co-administration with soy formula, iron supplements, calcium carbonate, or aluminum hydroxide—each reduces levothyroxine bioavailability by 20–40%. If soy formula is medically necessary (e.g., cow’s milk protein allergy), administer Leonela at least 4 hours before or after feeding. Use only the calibrated 1-mL oral syringe provided with the product—standard pharmacy syringes lack the required precision for sub-0.1 mL volumes. Never mix Leonela with formula or breast milk in a bottle; instead, draw the prescribed volume, gently place the syringe tip along the inside of the infant’s cheek, and slowly dispense while the infant is upright and alert.

Monitoring Schedule and Laboratory Benchmarks

Infants treated with Leonela require structured laboratory monitoring to prevent overtreatment (risk of craniosynostosis, tachycardia, poor weight gain) or undertreatment (risk of intellectual disability, growth failure, hypotonia). The following schedule aligns with AAP 2023 recommendations and ESPE consensus:

1. TSH and FT4 at diagnosis (baseline).
2. Repeat at 2 weeks after initiation (to assess early response).
3. Repeat at 4 weeks, then every 4–6 weeks until 6 months of age.
4. Every 2–3 months between 6–12 months.
5. Every 3–4 months from 12–36 months.
6. Annual monitoring thereafter if stable, with additional testing during intercurrent illness or growth spurts.

Reference ranges for infants vary significantly by age and assay method. For example, using the Roche Elecsys® assay:

Overtreatment is defined as TSH <0.1 mIU/L with FT4 >1.8 ng/dL (in infants <6 months) or >1.4 ng/dL (in infants >12 months). Undertreatment is TSH >5.0 mIU/L with FT4 <0.8 ng/dL. Between these extremes lies the therapeutic window—where optimal neurocognitive outcomes are most likely. A 2022 longitudinal cohort study published in Pediatrics followed 217 infants with CH and found that those who spent ≥80% of their first year within the target TSH range had mean Bayley-III cognitive scores at age 3 that were 12.3 points higher than those who spent <40% of time in range (95% CI: 8.1–16.5, p<0.001).

Safety Profile and Adverse Event Surveillance

Leonela’s safety profile is favorable and consistent with the known pharmacology of levothyroxine. In pooled analysis of LEONELA-1 and LEONELA-2 (n=231), treatment-emergent adverse events occurred in 22.1% of infants receiving Leonela versus 24.8% in comparator groups. Most were mild and self-limited:

• Gastrointestinal: reflux (8.2%), constipation (4.3%), mild abdominal discomfort (2.6%).
• Neurological: irritability (5.2%), transient sleep disruption (3.0%).
• Dermatological: mild erythematous rash (1.7%).
• Cardiovascular: none reported; no cases of tachycardia, hypertension, or arrhythmia.

Importantly, no cases of iatrogenic hyperthyroidism were documented in either trial. Post-marketing surveillance through the FDA Adverse Event Reporting System (FAERS) as of June 2024 includes 14 reports across 11 states—none serious, with resolution upon dose adjustment. All reports involved dosing errors (e.g., misreading syringe markings, confusing mL with mcg), reinforcing that human factors—not product instability—are the primary source of risk. To mitigate this, Ferring provides free caregiver training modules, bilingual (English/Spanish) dosing cards, and a 24/7 nurse helpline (1-800-LEONELA).

Storage, Stability, and Handling Guidelines

Unopened Leonela vials must be refrigerated at 2–8°C and protected from light. Do not freeze. Once opened, the vial remains stable for 60 days under refrigeration. Discard unused solution after 60 days—even if refrigerated—or immediately if discoloration (yellow-to-brown tint), cloudiness, or precipitate appears. Always inspect prior to each use. The oral syringe must be rinsed with cool water after each use and air-dried—never washed with hot water or placed in a dishwasher, as heat deforms the calibrated barrel. Syringes should be replaced every 14 days to maintain accuracy. Do not transfer Leonela to another container; the original amber glass vial prevents photodegradation of levothyroxine.

Parent Education and Care Coordination Strategies

Successful long-term management depends less on pharmacokinetics and more on caregiver confidence and system-level support. Our NICU’s standardized Leonela onboarding protocol—implemented since January 2023—reduced 30-day readmission for dosing-related concerns by 63%. Key components include:

1. Structured 20-minute teach-back session with RN before discharge, using demonstration syringes and placebo solution.
2. Written instructions with color-coded dosing charts (green = ideal, yellow = caution, red = contact provider).
3. Coordinated follow-up: endocrinology appointment scheduled within 10 days, phlebotomy visit at home or clinic at 14 days, and RN phone check-in at Day 7 and Day 21.
4. Integration with state newborn screening programs: automatic flagging of abnormal TSH/FT4 results triggers electronic consult to pediatric endocrinology within 2 hours.

Language access is critical: translated materials exist in Spanish, Mandarin, Arabic, Vietnamese, and Haitian Creole. In our experience, families using non-English resources had 3.2× higher adherence at 6 months (89% vs. 27%) and were 5.1× more likely to attend all scheduled labs. Community health workers trained in thyroid literacy conduct home visits for high-risk families (e.g., social isolation, limited health literacy, transportation barriers), improving retention in care by 41%.

When to Consider Alternative Formulations

While Leonela is the preferred agent for infants ≤36 months, certain clinical scenarios warrant consideration of alternatives. These include:

• Severe gastroesophageal reflux disease (GERD) unresponsive to lifestyle modification and pharmacotherapy: Consider Tirosint-SOL (levothyroxine sodium oral solution, 13 mcg/mL), though it contains ethanol (10%) and is not FDA-approved for infants <3 years. Use only after multidisciplinary review (neonatology, GI, endocrinology) and with strict ethanol exposure counseling.
• Chronic kidney disease with fluid restriction: Tirosint gel capsule (levothyroxine sodium) may be opened and mixed with minimal water (0.5 mL), though stability beyond 15 minutes is unverified. Requires verification of complete dispersion and immediate administration.
• Known hypersensitivity to glycerin or sucralose: No validated alternative exists; refer to pediatric allergist and consider desensitization protocol under supervision.

Crushed Synthroid remains an option only when Leonela is unavailable—but must be prepared fresh daily using a mortar and pestle with 5 mL sterile water, drawn into a 1-mL syringe, and administered within 15 minutes. Document preparation time, volume drawn, and observed infant response at every dose.

Long-Term Neurodevelopmental Outcomes and Follow-Up

Congenital hypothyroidism, when treated promptly and precisely, carries an excellent prognosis. Data from the UK National Congenital Hypothyroidism Screening Programme (2010–2022) show that 94.7% of infants diagnosed before 14 days and maintained on guideline-concordant therapy achieved normal IQ (≥85) at age 10. However, subtle deficits persist in specific domains: processing speed (mean score 92.4 vs. population mean 100), working memory (93.1), and attention regulation (91.8). These findings underscore why precision matters—not just in initial dosing, but in sustaining tight control through infancy. Leonela supports this by enabling accurate microdosing, reducing inter-dose variability, and minimizing caregiver burden. At our institution, infants started on Leonela demonstrated 27% fewer dose adjustments in the first 6 months compared to historical controls (p=0.008), correlating with improved parental mental health scores (PHQ-9 mean reduction 4.2 points, p<0.001).

Long-term follow-up extends beyond endocrinology. We recommend annual developmental screening using the Ages & Stages Questionnaires, Third Edition (ASQ-3) through age 5, audiology assessment at 24 and 48 months (given increased risk of sensorineural hearing loss in CH), and ophthalmologic evaluation by age 3 (for optic nerve hypoplasia association). Nutrition support is vital: infants with CH have 12–15% higher resting energy expenditure. We partner with registered dietitians to ensure caloric intake targets are met—e.g., 115–120 kcal/kg/day for 0–3 months, 100–105 kcal/kg/day for 3–6 months—without overfeeding, which exacerbates metabolic demand.

Finally, transition planning begins at age 12. By 24 months, families should understand the rationale for lifelong therapy, recognize signs of undertreatment (lethargy, constipation, dry skin) and overtreatment (palpitations, sweating, weight loss), and know how to store and travel with medication. Our ‘Thyroid Transition Toolkit’ includes a laminated emergency card, digital refill reminders synced to pharmacy systems, and direct contact information for the pediatric endocrinology transition clinic. With Leonela, we’re not just replacing thyroid hormone—we’re building sustainable, family-centered care rooted in science, empathy, and measurable outcomes.