THE IMPORTANCE OF THE MOTHER AS A BIOASSAY FOR DIAGNOSING AND TREATING FOETAL THYROTOXICOSIS
ST Wahid1, R Jha2, K Brown3 & JU Weaver1
The foetus of a mother with previous or current Graves disease is at risk of developing thyrotoxicosis. The mother can act as a bioassay to diagnose foetal thyrotoxicosis. We describe the difficulties in managing foetal thyrotoxicosis in a mother who could not act as a bioassay because of previous Graves disease treated by total thyroidectomy.
A 35year old woman fell pregnant 3 months after a total thyroidectomy for relapsed Graves disease and concomitant eye disease. Her thyroxine dose was increased from 150 to 250 microg o.d over the first 12 weeks to maintain euthyroidism throughout pregnancy. At 17 weeks gestation (17/40) a TSH-receptor binding inhibitory immunoglobulin (TBII) level of 58 u (normal <15 u) suggested that the foetus had a high risk of developing thyrotoxicosis. TBII rose to 70 u by 25/40. Thereafter bi-weekly TBII, weekly foetal heart rate (FHR) monitoring via cardiotocography and monthly foetal biophysical profiles were done. Up to 30/40 FHR remained less than 160 bpm, TBII varied from 66-73 u and foetal growth was satisfactory. At 31/40 a FHR of 166 bpm suggested foetal thyrotoxicosis. The mother was administered propylthiouracil (PTU) 50mg t.d.s. At 33/40 FHR was still high at 151 bpm, requiring PTU 100mg t.d.s. Until delivery FHR remained controlled at 124-140 bpm and TBII varied from 64-73 u. A 7lb-baby boy with a TSH of 61 mu/L (normal 1.0-9.1 mu/L) was delivered. Thyrotoxicosis developed 5 days later (TSH=0.41 mu/L, free T4=71.9 pmol/L (normal 9-40 pmol/L)), which required treatment with propranolol and PTU until neonatal TBII levels had fallen from 79 to less than 5 u 8 weeks after birth.
Our case illustrates the difficulty in maintaining euthyroidism in foetal thyrotoxicosis despite TBII and FHR measurements. Total thyroidectomy in women of childbearing age with Graves disease should be undertaken as a last resort.