Endocrine Abstracts

Mrs X (aged 79) had a history of longstanding hypothyroidism (levothyroxine 100 micrograms daily). Her Free T4 (FT4) had been slightly raised with normal TSH until in Dec 2019, where both became elevated (TSH 5.4 mU/l and FT4 29.4 pmol/l). Repeat TFT in January 2020 also showed the same. Mrs X felt well on Levothyroxine, with no thyrotoxic features. Endocrine clinic investigations (October 2020) showed no evidence of assay interference. Alternative diagnoses such as thyrotropinoma (TSHoma) or thyroid hormone resistance were considered. Mrs X was advised to stop levothyroxine in September 2021. Anterior pituitary hormones measured seven weeks later were unremarkable (FSH 69.6 IU/l, LH 33 IU/l, prolactin 498 mIU/l, negative macroprolactin, IGF 17.2 nmol/l and cortisol 672 nmol/l). Alpha subunit was raised at 19.85 IU/l. Contrast Pituitary MRI was unremarkable. Thyrotropin-releasing hormone (TRH) stimulation test showed a brisk TSH response to TRH stimulation (baseline TSH 15.8 mU/l, 30 minutes 44.9 mU/l, 60 minutes 39.4 mU/l), suggesting either hypothyroidism or thyroid hormone resistance. Mrs X felt tired and struggled to function off levothyroxine. Her TSH was 16.9 mU/l and FT4 was 34 pmol/l whilst off levothyroxine. She was restarted on levothyroxine 50 micrograms daily in December 2021. Repeat TFT 1 month later showed TSH of 4.3 mU/l, and FT4 of 57.4 mU/l. She remained clinically well with no thyrotoxic features. Nevertheless, a repeat alpha-subunit in January 2022 was 23.30 IU/l. Due to progressive weight loss, CT chest-abdomen-pelvis performed showed metastatic pancreatic cancer. She is currently receiving palliative chemotherapy from the oncology team. Whether this is coincidental to her abnormal TFTs is speculative. However, this case highlights the need to consider occult malignancy where no clear reason for new thyroid hormone resistance can be found.