Possible explanation | Clinical clues/investigation | Next steps |
Pattern 1: fT4 in the upper one-half of the reference range, and TSH mildly elevated (eg, 5 to 20 mU/L) | ||
Child was given extra doses of levothyroxine just before scheduled blood test monitoring, to make up for missed doses | Parents/caregivers confirm that they gave extra doses before testing |
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Mild underdosing* | Parents/caregivers report that they did not give extra doses before testing |
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Thyroid hormone resistance¶ | Previous dose increases did not normalize TSH and/or child has signs of thyrotoxicosisΔ |
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Pattern 2: fT4 elevated, and TSH in the normal reference range | ||
The elevated fT4, though higher than this patient's "physiologic set point," is required to generate normal T3 levels◊ | Review diagnostic evaluation for congenital hypothyroidism, including thyroid ultrasound results |
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The fT4 reference range reported for this assay is not appropriate for neonates or infants, who tend to have higher normal reference ranges | Review reference range for the neonate's or infant's age (refer to UpToDate content on diagnosis of congenital hypothyroidism) |
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The fT4 is normal for this patient because their "physiologic set point" is slightly above the upper end of the listed reference range | This explanation is likely if fT4 elevation is mild and persistent and if the above causes are excluded |
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fT4: free thyroxine; TSH: thyroid-stimulating hormone; T3: triiodothyronine.
* Mild underdosing occasionally causes this "disparate" pattern of mildly elevated TSH with normal fT4. More commonly, underdosing causes elevated TSH with low or low-normal fT4.
¶ Many infants have transient mild thyroid hormone resistance during infancy (due to resetting of the pituitary-thyroid feedback threshold because of intrauterine hypothyroidism). The pattern is more common in infants than in older children (43 versus 10%, respectively, in 1 study)[1].
Δ Symptoms and signs of thyrotoxicosis in an infant include tachycardia, stare, irritability, hyperactivity, restlessness, and poor sleep.
◊ One example of an altered set point is an individual with thyroid aplasia who produces 20% less T3 compared with those with a normal thyroid gland.
§ In these scenarios, the elevated fT4 does not indicate overtreatment, because TSH is normal.Do you want to add Medilib to your home screen?