Endocrine Abstracts

The differential diagnosis of post-partum thyrotoxicosis includes Graves' disease and post-partum thyroiditis(PPT). We present two cases which demonstrate the difficulty of interpreting 99mTc-pertechnetate scans in this setting. Case 1 is a 37 year-old lady who presented 5 months post-partum with a month history of palpitations, sweating and heat intolerance. Investigations revealed TSH < 0.01 mU/L, fT3 8.9 pmol/L (NR 10.5-22.7 pmol/L) and fT4 23.4 pmol/L (NR 3.5-6.5 pmol/). A pertechnetate scan demonstrated reduced homogenous uptake in the thyroid at 0.5%. Subsequently the patient became hypothyroid without anti-thyroid medication in keeping with the clinical diagnosis of PPT. Case 2 is a 37 year-old lady with a previous history of Graves' disease who presented 3 months post-partum with increased agitation and sweating. Investigations revealed TSH < 0.01 mU/L, fT3 15.6 pmol/L and fT4 48 pmol/L. A pertechnetate scan demonstrated patchy reduced uptake into the thyroid at 0.4%. Thyroid peroxidase antibodies were raised at 178 iU/ml (NR < 60) with negative TSH receptor stimulating antibodies. Subsequently the patient remained thyrotoxic over several weeks and responded to propylthiouracil, confirming the working diagnosis of Graves'. Both patients in whom the very low tracer uptake values did not differentiate between the two distinct clinical conditions were lactating at the time and scintigraphy revealed very avid breast uptake. The activation of the sodium iodide symporter (NIS), an intrinsic plasma membrane protein that mediates the active transport of iodide and similar ions in the thyroid gland and a number of extrathyroidal tissues, in particular lactating mammary gland, is well described in literature. This must competitively reduce thyroid uptake of tracer to the lower end of the normal predicted values. The learning point of these cases is that the time honoured technique of thryroid scintigraphy may not reliably differentiate between Graves and PPT in the lactating patient.