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Free thyroid hormones in evaluating persistently elevated thyrotropin levels in children with congenital
Insights
In children with congenital hypothyroidism, elevated TSH despite normal total thyroxine (T4) indicates inadequate levothyroxine (L-T4) therapy. Monitoring TSH and free hormone levels is crucial for effective treatment.
Area of Science:
- Pediatric Endocrinology
- Thyroid Disorders
- Pharmacological Monitoring
Background:
- Congenital hypothyroidism requires levothyroxine (L-T4) replacement therapy.
- Some children exhibit elevated thyroid-stimulating hormone (TSH) despite normal total thyroxine (T4) levels on L-T4 therapy.
- This may suggest altered TSH feedback regulation thresholds.
Purpose of the Study:
- To investigate serum free thyroxine (FT4) and free triiodothyronine (FT3) in L-T4 treated hypothyroid children with varying TSH levels.
- To assess the reliability of total T4 versus free hormone levels in monitoring L-T4 therapy adequacy.
Main Methods:
- Serum TSH, FT4, and FT3 levels were measured in two groups of L-T4 treated hypothyroid children (high TSH vs. normal TSH) and a control group.
- Patients were clinically assessed for euthyroid status.
- Age-matched groups were compared.
Main Results:
- Children with high TSH (Group A) had significantly lower FT4 and FT3 compared to those with normal TSH (Group B).
- Group B showed higher FT4 and T4, and lower TSH compared to controls (Group C).
- The T4/T3 ratio was elevated in both hypothyroid groups compared to controls.
Conclusions:
- Elevated TSH in L-T4 treated children often signifies inadequate therapy, detectable by low free hormone levels, not normal total T4.
- Monitoring TSH and free hormone concentrations is recommended for optimal L-T4 therapy.
- Free hormone levels can also detect subclinical overdosage.
Abstract:
Some children with congenital hypothyroidism receiving L-T4 therapy have elevated serum TSH levels despite having normal serum T4 concentrations, suggesting that they have a higher threshold for the feedback regulation of TSH release. To further study this possibility, we determined serum free T4 (FT4) and T3 (FT3) concentrations in two groups of L-T4-treated hypothyroid children. Group A consisted of 10 patients with high serum TSH levels; group B consisted of 10 patients with normal TSH levels. All patients were clinically euthyroid, and serum total T4 and T3 concentrations were similar in the two groups. A third (control) group (C) consisted of randomly selected normal children. The three groups were age matched. Serum FT3 and FT4 were significantly lower in group A compared to group B. Serum FT4 and T4 were higher and TSH was lower in group B compared to group C. The T4/T3 ratio wash higher in both groups of children with hypothyroidism than in group C. We conclude that in most patients a high serum TSH was due to inadequate L-T4 therapy, as shown by free hormone concentrations (low) but not by total hormone levels (normal). This suggests that L-T4 therapy should be monitored by measurement of TSH and free hormone concentrations. The latter also can be used to indicate moderate overdosage, not clinically detectable, as shown by the comparison between groups B and C. Measurement of serum total T4, as indicated by the lack of difference between groups A and B and also by T4/T3 ratio, cannot be considered a reliable index of therapeutic adequacy in such children.