NN Ttoxicosis

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Neonatal thyrotoxicosis (hyperthyroidism in a newborn) is uncommon but important to recognize quickly because it can affect the baby’s heart, growth, and brain. In the UK, clinicians usually look at free T4 (FT4) and TSH levels together rather than a single cutoff.

Typical lab pattern

In neonatal thyrotoxicosis you usually see:

• Free T4: elevated above the normal neonatal range

• TSH: very low or suppressed

Approximate UK reference ranges

(Newborn ranges vary slightly by lab and by the baby’s age in days.)

Test	Typical newborn reference	Concerning for thyrotoxicosis
Free T4 (FT4)	~10–30 pmol/L	Often >30–40 pmol/L
TSH	~0.5–10 mU/L (varies after birth)	Very low / <0.1 mU/L

In many confirmed neonatal thyrotoxicosis cases:

• FT4 may be 40–80 pmol/L or higher, with

• TSH suppressed.

Important clinical context

Doctors also consider symptoms such as:

• fast heart rate (tachycardia)

• irritability

• poor weight gain

• warm skin / sweating

• enlarged thyroid (goitre)

Most neonatal cases occur when the mother has Graves’ disease, because thyroid-stimulating antibodies cross the placenta.

Important note

Diagnosis in the UK is never based on T4 alone. Paediatricians assess:

• FT4

• TSH

• TSH receptor antibodies (TRAb)

• clinical signs in the baby.

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✅ If you want, I can also explain:

• the exact NHS diagnostic criteria, or

• how doctors treat neonatal thyrotoxicosis (carbimazole, propranolol, etc.).