Endocrine Abstracts

A 33-year-old woman was seen in the Endocrinology Clinic in May 2023 due to abnormal thyroid function tests with normal TSH and persistently elevated free T4 and free T3. She had been under investigation for tiredness, palpitations, and weight gain. The differential diagnosis was TSH-secreting tumour or thyroid hormone resistance. Her bloods were sent to Basingstoke Hospital and assay interference by heterophilic antibodies was excluded. She underwent a thyroid ultrasound which revealed a mildly enlarged gland, with normal reflectivity and slightly coarse echotexture. Between her appointments, she suffered from palpitations, dizziness, and collapses. The Cardiology team organised a 24-h Ambulatory ECG monitoring which showed 42 episodes of sinus tachycardia and a transthoracic echocardiogram which was unremarkable. To manage her symptoms, propranolol 160mg MR OD was commenced.

Her thyroid functions tests were as below:

She had an MRI of her pituitary with gadolinium in June 2023, revealing no pituitary lesion. She then underwent a TRH stimulation test in August 2023. Her TSH went up from 2.24 at 0 minutes to 20.59mu/l at 20 minutes post TRH stimulation. It then dropped to 11.39 mu/l 60 minutes following the TRH administration, a response consistent with thyroid hormone resistance. In contrast, the pituitary response is blunted in the case of a TSH-oma. Genetic testing followed which revealed a causal mutation (Ala279Val) in the Thyroid Receptor Beta gene. This leads to insufficient negative feedback on the pituitary but a normal response to thyroid hormones by peripheral tissues. The patient is heterozygous and inheritance is autosomal dominant. Treatment with triiodothyroacetic acid was proposed to suppress TSH as it does not act on peripheral tissues and cannot have thyrotoxic effects. In addition, the patient will receive cardioselective Beta-blockers.