Endocrine Abstracts

Objective

Thyroid replacement therapy needs careful titration, and recently there has been discussion regarding the bioavailability of different thyroxine preparations [1]

We have taken the opportunity of comparing thyroid function in 18 females who were enrolled in a trial studying the effects of thyroxine therapy on subclinical hypothyroidism [SCH].

Methods

These patients (at entry to trial, mean age 50.1 plus/minus 4.1 yrs, mean T4 12.2 plus/minus 0.2 picomol per litre [NR 9.8-22.1], median TSH 8.8 milliunits per litre [NR 0.35-5.5] had been taking Synthroid (Abbott laboratories) for 6 months, the dose had been titrated from 50 to a maximum 150 micrograms, and for the last 12 weeks of the trial each patient was stable on the same dose of Synthroid [mean daily dose 112.5micrograms].

Each patient was then prescribed a British thyroxine formulation (Forly generics, CP Pharmaceuticals) at the same dose and thyroid function was measured at 6 and 12 weeks.

Results

After 12 weeks of Synthroid treatment mean free T4 was 18.6 plus/minus 2.5 picomol per litre. After 6 weeks of generic thyroxine treatment FT4 fell to 16.8 picomol per litre and at twelve weeks was 17.03 picomol per litre. TSH however doubled from 0.72 milliunits per litre to 1.59 milliunits at 6 weeks and 1.57 milliunits per litre at 12 weeks. 11% of patients at 12 weeks showed a TSH that was higher than the normal range and two patients showed Free T4 levels above the normal range.

Conclusion

The increase in TSH and abnormalities of increased freeT4 levels after conversion in this study demonstrates the importance of evaluating thyroid hormone levels whilst on different thyroid hormone replacement therapies and very careful dosing is needed to avoid over or under replacement and associated co-morbidities.

Reference

[1] Blakesley V et al, Thyroid:2004 Mar;14(3):191-200