Case
A 34-year-old primigravida was referred to a tertiary diabetes in pregnancy clinic. At presentation, the patient had a 20-week OGTT demonstrating F: 5.7mmol/L, 1h 10.7mmol/L and 2h 8mmol/L with HbA1c of 5.5%. The patient had no known pre-existing diabetes. The patient had documented evidence of impaired fasting glucose throughout her childhood and adolescence (range 5.5-6.2mmol/L). She did not present to hospital previously with any metabolic complications, including diabetic ketoacidosis. She rapidly transitioned to basal-bolus (Humalog and Levemir) insulin at 20 weeks of gestation. Despite this, the patient was challenged with labile blood sugar readings between weeks 30 and 38 gestation. There was an overall trend towards insulin sensitivity, and she would encounter significant hyperglycaemic and hypoglycaemic events despite maintaining constant carbohydrate intake and administering small doses of insulin.
The patient’s anthropometry revealed BMI of 20.6kg/m2. There was no clinical evidence of macrovascular or microvascular complications. Her blood tests from February 2024 revealed a HbA1c of 5.5%. We conducted exocrine pancreas enzyme and antibody testing (IA2, GAD, Zn transporter 8) and all results were negative on repeated testing. A paired blood test in February 2024 revealed a glucose of 6.2mmol/L and a corresponding C-peptide of 76 umol/L. Genetic testing did not reveal any monogenic aetiology. In the post-partum phase, the patient was observed for a period of 24 hours without any insulin and recorded high fasting, random, and post-prandial blood glucose measurements, were all consistent with a diagnosis of diabetes mellitus. She was transitioned to back on Humalog and Levemir post-partum. To date, her blood sugar profile remains stable on this insulin regimen, and she will continue to be reviewed in the pre-pregnancy clinic with serial testing of diabetes auto-antibodies.
Discussion
Diagnosis of type 1b diabetes remains rare, particularly within pregnancy. We did not identify any published case reports demonstrating this clinical phenomenon. Most case reports of type 1b (non-autoimmune) diabetes were diagnosed in the presence of exocrine pancreas dysfunction.1 Whilst pregnancy itself may, in certain cases, confer suppression of autoimmunity, whether this correlates to an absence of circulating type 1 diabetes autoantibodies remains unlikely.2 This case represents important challenges for future pregnancy planning and intrapartum management of glycaemic control.
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