The prognosis of primary congenital hypothyroidism, which has potentially devastating effects on neurodevelopment, has been almost eradicated after the implementation of the neonatal screening programme. Although early facilitation with substitution of levothyroxine is preventative, caution should still be taken in severe cases, as subtle psychomotor delays may occur. Treatment dose may be indicative of the severity and pathology behind the condition and can be used as guidance when deciding which patients should have a trial of levothyroxine in cases where the thyroid gland remains in situ.