Endocrine Abstracts

Introduction: The diagnosis of central hypothyroidism can be challenging and confused with other causes of low thyroxine concentrations without TSH elevation. The response of the thyroid hormone axis before and after treatment is reported in children with Central Hypothyroidism [CH] as part of an exercise aimed at validating free thyroxine reference ranges.

Methods: Roche Elecsys Cobas TSH and FT4 immunoassays were used. TSH and free T4 (FT4) concentrations were reported in children (n=18) with confirmed CH before and after treatment. Confirmation of the diagnosis was made by finding MRI abnormalities [4 children] or the presence of other pituitary hormone abnormalities [15 children]. The age range of patients was from <1 month to 17 years. Mean (±S.D.) values are quoted for normally distributed data; otherwise median (range) values are quoted, with median groups compared using the Kruskall Wallis test.

Results: At diagnosis the mean FT4 concentration was 8.4 (± 2.0) pmol/L. Two children were above the lower FT4 cut-off of 10.8 pmol/L. Mean TSH concentration was 3.2 (± 2.2) mU/L where only 1 child had an undetectable TSH, 15 children were within their age-related reference range and 2 children were above. There was no correlation between FT4 and TSH concentrations prior to treatment. After treatment the median FT4 concentration was 16.0 (12.7 to 18.7) pmol/L. TSH concentration was suppressed from the pre-treatment concentration in 6 children and was below the reference range in 4. The post treatment TSH clearly fell into two groups (P<0.001): those with a normal TSH concentration and those in whom the TSH was suppressed.

Conclusions: In Central Hypothyroidism at diagnosis, free thyroxine concentrations may overlap with the lower part of the reference range and the TSH may be low, normal or slightly raised. It is rarely undetectable. After treatment the TSH suppresses indicating that the feedback control loop is still active in CH. However, CH patients clearly separate into two groups: those with detectable and those with undetectable TSH concentrations on treatment. This may reflect the differing pathological causes of the CH and may indicate a new way of classifying Central Hypothyroidism.