SFEEU2017 Clinical Update Workshop C: Disorders of the thyroid gland (6 abstracts)
We report a 55 year old non-smoker with a history of Graves disease diagnosed in 1990 at age 29 years old. Due to poor compliance to therapy, she underwent a total thyroidectomy within 1 year of diagnosis. She remained well controlled on thyroid hormone replacement for over 20 years with Levothyroxine 100 μg daily. However, in the 2 years before referral to the Endocrine Clinic, she had difficult to treat hypothyroidism with persistent over-replacement; at the time of referral she was taking Levothyroxine 25 μg daily. She reported no symptoms of over-replacement. She had no evidence of thyroid associated orbitopathy. Taking Levothyroxine 25 μg daily, TSH remained suppressed at <0.01 mIU/l, Free T4 14.5 pmol/l and Free T3 4.3 pmol/l. Thyroid autoantibodies were both positive: thyroid peroxidase 285 unit/mL and TSH receptor 2.9 unit/ml. Ultrasound of the thyroid bed confirmed three hypervascular thyroid nodules measuring 8, 27 and 16 mm. Thyroid hormone replacement was discontinued to facilitate a thyroid uptake scan; this will likely be followed by radioactive iodine therapy if she has ongoing biochemical subclinical thyrotoxicosis.
Graves disease is an autoimmune condition characterised by the production of autoantibodies against the thyroid-stimulating hormone receptor: TSH-receptor antibodies (TRAb). TRAb stimulates target organs with the majority developing hyperthyroidism from stimulation of follicular cell production of thyroid hormone and about half developing thyroid associated orbitopathy. Total thyroidectomy removes target tissue for TRAb and controls hyperthyroidism. Surgical thyroid resection is usually followed by a reduction of TRAb levels in variable degrees; the degree of reduction remains controversial. The median TRAb value half-life has been estimated at 93.5 days after total thyroidectomy in patients without orbitopathy or smoking, such as our patient. This case therefore demonstrates that remnant thyroid cells after total thyroidectomy in Graves disease can become stimulated under the mediation of TRAb causing recurrent thyrotoxicosis. Revision surgery would be challenging and therefore radioactive iodine necessary.