Predicting thyroxine requirements following total thyroidectomy

Authors


Mr Dipan Mistry, c/o secretary to Mr England, ENT Department, Castle Hill Hospital Cottingham, Hull, HU16 5JQ, UK. Tel.: +44 (0)1482 624781; E-mail: dipanmistry@hotmail.com

Summary

Objective  Optimal thyroxine replacement following total thyroidectomy is critical to avoid symptoms of hypothyroidism. The aim of this study was to determine the best formula to determine the initiated replacement dose of levothyroxine immediately following total thyroidectomy.

Design  Prospective study. All patients were initiated on 100 μg levothyroxine and titrated to within the reference range for TSH and free T4. Correlations to height, weight, age, lean body mass (LBM), body surface area (BSA) and body mass index (BMI) were calculated.

Patients  One hundred consecutive adult patients underwent total thyroidectomy for non-malignant disease.

Measurements  Comparison between three methods of levothyroxine dose prediction, aiming for a levothyroxine dose correct to within 25 μg of actual dose required.

Results  Correlations were seen between levothyroxine dose and patient age (r = −0·346, P < 0·01), bodyweight (r = 0·296, P < 0·01), LBM (r = 0·312, P < 0·01), BSA (r = 0·319, P < 0·01) and BMI (r = 0·172, P < 0·05). A regression equation was calculated (predicted levothyroxine dose = [0·943 × bodyweight] + [−1·165 × age] + 125·8), simplified to (levothyroxine dose = bodyweight − age + 125) pragmatically. Initiating patients empirically on 100 μg post-operatively showed that 40% of patients achieved target within 25 μg of their required dose; this increased to 59% when using a weight-only dose calculation (1·6 μg/kg) and to 72% using the simplified regression equation.

Conclusions  A simple calculated regression equation gives a more accurate prediction of initiated levothyroxine dose following total thyroidectomy, reducing the need for outpatient attendance for dose titration.

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