Hipotiroidisme Kongenital

01Pathophysiology

Congenital hypothyroidism (CH) is the most common congenital endocrine disorder and the most common preventable cause of intellectual disability. This last phrase is the single most tested association, and it underscores why newborn screening programs exist worldwide.

Thyroid hormone is essential for central nervous system myelination and maturation during the first 2-3 years of life. The fetal thyroid gland begins producing hormones by approximately 12 weeks of gestation, and the hypothalamic-pituitary-thyroid (HPT) axis becomes functional by mid-gestation. Before that point, the fetus depends entirely on maternal T4 crossing the placenta. This is why many affected neonates appear clinically normal at birth: residual maternal hormone provides a temporary buffer. However, once that supply is exhausted postnatally, the infant becomes progressively hypothyroid.

The most common cause of permanent CH is thyroid dysgenesis, which encompasses a spectrum of developmental anomalies. The thyroid gland originates at the base of the tongue (foramen cecum) and descends to its normal pretracheal position during embryogenesis. When this migration is incomplete or the gland fails to form properly, you get ectopic thyroid tissue (most common subtype of dysgenesis), thyroid agenesis (complete absence), or thyroid hypoplasia (underdeveloped gland). Ectopic tissue, often found at the lingual or sublingual position, typically produces insufficient hormone.

The second major category is dyshormonogenesis, which results from inherited enzymatic defects in thyroid hormone synthesis. These are autosomal recessive conditions. The most commonly tested enzyme defect involves thyroid peroxidase (TPO), which is needed for iodide organification. When TPO is deficient, iodide cannot be incorporated into thyroglobulin, and T3/T4 production fails. Because the gland itself is present and responds to chronically elevated TSH with compensatory growth, patients with dyshormonogenesis may present with a goiter, which is a distinguishing feature from thyroid dysgenesis where no gland (or only ectopic tissue) is palpable.

Central (secondary or tertiary) congenital hypothyroidism is rare and results from pituitary or hypothalamic dysfunction. The critical exam point here is that TSH-based newborn screening programs will miss these patients because their TSH is low or inappropriately normal while free T4 is low. This is why some screening programs use a combined TSH + T4 approach.

The downstream effects of hypothyroidism on the neonate directly explain the clinical findings. Decreased metabolic rate leads to hypothermia, constipation, and lethargy. Reduced bilirubin conjugation (from decreased hepatic UDP-glucuronosyltransferase activity) causes prolonged unconjugated hyperbilirubinemia. Decreased collagen turnover and mucopolysaccharide accumulation in tissues produce the coarse facial features, macroglossia, and myxedema. Delayed skeletal maturation results in large fontanelles and, if untreated, short stature with epiphyseal dysgenesis.

02Classification and Clinical Manifestation

By Etiology

By Timing

03Diagnostic Workup

The newborn screening test is the best initial test and is the most commonly tested concept. Most programs worldwide use a primary TSH strategy: a heel-prick blood sample obtained between 24-72 hours of life measures TSH. An elevated TSH triggers recall for confirmatory venous testing. Some programs use a primary T4 strategy (measures T4 first, with reflex TSH if T4 is low), which has the advantage of detecting central hypothyroidism but the disadvantage of missing mild or compensated primary disease.

When the screening result is abnormal, the confirmatory workup is a venous serum TSH and free T4. In primary CH, TSH is elevated (often markedly, >40-50 mIU/L) and free T4 is low. In central CH, both TSH and free T4 are low (TSH may be low-normal or inappropriately normal for the degree of T4 deficiency).

A critical exam point: do not delay treatment while waiting for imaging. Treatment with levothyroxine must begin as soon as confirmatory serum results indicate CH. Imaging studies to determine etiology can be performed concurrently or deferred.

Thyroid ultrasound is noninvasive and identifies whether a thyroid gland is present in the normal position, is ectopic, or is absent. It is particularly useful for identifying thyroid tissue but may miss small ectopic deposits. Radionuclide scintigraphy (Tc-99m pertechnetate or I-123) is the gold standard for localizing functional thyroid tissue. If uptake is absent everywhere, the diagnosis is thyroid agenesis (confirmed by an undetectable thyroglobulin level). If uptake appears in an abnormal location (e.g., sublingual), the diagnosis is ectopia. If there is increased, diffuse uptake in a normally positioned or enlarged gland, the diagnosis is dyshormonogenesis (the gland is present but enzymatically defective, and TSH-driven stimulation causes increased trapping).

A knee radiograph looking for the distal femoral epiphysis is occasionally tested. This ossification center is normally present at term (36-40 weeks). Its absence in a term neonate suggests that the hypothyroidism was severe and present during the third trimester, indicating more significant in-utero deficiency and potentially worse neurodevelopmental prognosis if treatment is delayed.

04Management and Treatment

Levothyroxine (L-T4) is the only accepted treatment. Triiodothyronine (T3) preparations and combination T4/T3 therapy are not recommended in neonates. The starting dose for a full-term neonate is 10-15 mcg/kg/day, which translates to approximately 37.5-50 mcg/day for an average 3.5 kg newborn. Some guidelines recommend starting at the higher end (15 mcg/kg/day) for severe cases (e.g., very low initial T4, absent gland on imaging, absent knee epiphyses) to achieve faster normalization.

The tablet should be crushed and dissolved in a small volume of breast milk or water and given orally via syringe. It must NOT be mixed with soy-based formula, calcium, or iron supplements, as these interfere with intestinal absorption of levothyroxine. Administration should be at a consistent time each day, ideally 30 minutes before feeding.

The treatment goal is to normalize free T4 into the upper half of the reference range within 2 weeks and to bring TSH below 5 mIU/L (ideally 0.5-2.0 mIU/L) within the first month. If TSH remains elevated despite adequate dosing, assess adherence and absorption before increasing the dose.

Dose adjustments are frequent in the first few years of life because of rapid growth. Underdosing risks neurocognitive impairment; overdosing risks craniosynostosis, temperament changes, and accelerated bone age. This is why close monitoring with serum levels is essential.

For patients with uncertain etiology (e.g., no imaging was performed at diagnosis, or the screening result was only mildly abnormal), a re-evaluation trial is recommended at approximately 3 years of age. By this age, the critical window for thyroid-dependent brain development has closed, so a brief period off medication is safe. The levothyroxine dose is either reduced by 30% or discontinued entirely for 30 days, followed by repeat TSH and free T4 measurement. If TSH rises and free T4 drops, the diagnosis of permanent CH is confirmed and lifelong therapy continues. If levels remain normal, the condition was transient and medication can be stopped.

Contraindication awareness: Levothyroxine itself has no absolute contraindications in neonates. However, the exam may test your knowledge that excessive dosing in infants can cause irritability, tachycardia, diarrhea, and premature craniosynostosis. The key principle is to dose aggressively enough to prevent neurodevelopmental damage but to monitor closely to avoid supratherapeutic levels.

05Differential Diagnosis and Distractors

06Traps and High-Yield Pearls

The most common trap on this topic is the student who encounters a neonate with prolonged jaundice, poor feeding, and lethargy and jumps to a diagnosis of sepsis or breastfeeding failure without considering hypothyroidism. The vignette will often include a subtle clue, such as a large posterior fontanelle, constipation, or a note that the infant is post-term or large for gestational age, that should redirect your thinking toward an endocrine cause.

A second classic trap involves the question "What is the most important next step?" when presented with an abnormal newborn screening result. The correct answer is always confirmatory serum TSH and free T4, followed immediately by starting levothyroxine. Students sometimes choose imaging (ultrasound or scintigraphy) as the next step, but imaging determines etiology, not diagnosis, and must never delay initiation of treatment.

Another frequently tested point is the association between CH and the phrase "most common preventable cause of intellectual disability." If a question stem asks about the most common preventable cause of mental retardation or cognitive impairment, the answer is congenital hypothyroidism, not phenylketonuria (which is the second most common). Both are detected by newborn screening, but CH is more prevalent (approximately 1 in 2,000-4,000 live births versus 1 in 10,000-15,000 for PKU).

Finally, be aware of the soy formula trap. If a vignette describes an infant with CH on levothyroxine who has persistently elevated TSH despite appropriate dosing, look for a mention that the child is fed soy-based formula. Soy protein impairs the intestinal absorption of levothyroxine, requiring a dose increase or a switch to a different formula. Iron and calcium supplements similarly reduce absorption and are tested in the same pattern.

The core competency being tested across all question formats on CH is the ability to recognize an often-subtle neonatal presentation, to understand the urgency of early treatment (within 2 weeks of life) for neurodevelopmental protection, and to correctly sequence the diagnostic and management steps without unnecessary delay.