Most Likely Diagnosis: Type 1 Diabetes Mellitus with Concurrent Primary Hyperparathyroidism
The most likely diagnosis is Type 1 Diabetes Mellitus (Option A), with the hypercalcemia and elevated PTH representing a separate, concurrent condition of primary hyperparathyroidism.
Diagnostic Reasoning
Evidence Supporting Type 1 Diabetes Mellitus
The patient's presentation strongly indicates Type 1 DM based on multiple converging factors:
Classic symptoms of polyuria and polydipsia are hallmark presentations of Type 1 diabetes, particularly in patients presenting with new-onset hyperglycemia 1
Low C-peptide level (0.09) is the critical distinguishing feature that confirms insulin deficiency due to beta-cell destruction, as C-peptide testing distinguishes Type 1 from Type 2 diabetes 1
Positive family history of Type 1 DM increases the likelihood of autoimmune diabetes, as Type 1 diabetes has genetic predisposition 1
HgA1c of 8.5% confirms diabetes diagnosis (≥6.5% diagnostic threshold) and indicates significant hyperglycemia 1
Why Not the Other Options
Type 2 DM (Option B) is excluded because Type 2 diabetes is characterized by insulin resistance with preserved or elevated C-peptide levels, not the severely low C-peptide (0.09) seen in this patient 1. The low C-peptide definitively indicates insulin deficiency from beta-cell destruction, not insulin resistance 1.
Central Diabetes Insipidus (Option C) is excluded because this condition does not cause hyperglycemia or elevated HgA1c. Central DI causes polyuria/polydipsia through impaired ADH secretion, but blood glucose remains normal 1.
MODY (Option D) is less likely because MODY typically presents with mild hyperglycemia in young patients with strong multigenerational family history, and usually maintains some C-peptide production. The severely low C-peptide and family history specifically of Type 1 DM (not Type 2 or MODY) point away from this diagnosis 1.
The Concurrent Hyperparathyroidism
The elevated PTH and calcium represent primary hyperparathyroidism occurring simultaneously with Type 1 diabetes:
Elevated or inappropriately normal PTH with hypercalcemia confirms primary hyperparathyroidism as a separate diagnosis 2, 3
The association between diabetes and hyperparathyroidism is well-documented, with prevalence of diabetes in primary hyperparathyroidism approximately 8%, and prevalence of hyperparathyroidism in diabetic patients approximately 1%—both about three-fold higher than expected in the general population 4, 5
Both disorders can present together or within 1 year in approximately 40% of cases where they coexist 4
Clinical Significance of the Dual Diagnosis
Hyperparathyroidism can worsen glucose control through insulin resistance caused by elevated intracellular calcium, which decreases insulin-stimulated glucose transport 4, 6
Hypercalcemia symptoms (polyuria, polydipsia) overlap with diabetes symptoms, but the elevated calcium and PTH clearly indicate a separate parathyroid pathology 2, 7
Parathyroidectomy may improve glucose control in 37-77% of diabetic patients with concurrent hyperparathyroidism, making early diagnosis clinically important 6
Management Implications
Immediate priorities:
Confirm Type 1 diabetes with autoantibody testing (GAD65, anti-insulin, anti-islet cell, ZnT8) to document autoimmune beta-cell destruction 1
Initiate insulin therapy immediately as Type 1 diabetes requires exogenous insulin due to absolute insulin deficiency 1
Evaluate hyperparathyroidism for surgical candidacy, as corrected calcium >1 mg/dL above upper limit of normal is a surgical indication 2
Obtain parathyroid imaging (ultrasound and/or Tc-99m sestamibi scan) for surgical planning once biochemical diagnosis is confirmed 2, 5
Common Pitfall to Avoid
Do not assume the hypercalcemia is causing the diabetes symptoms alone—the severely low C-peptide definitively indicates Type 1 diabetes requiring insulin therapy, not just metabolic derangement from hypercalcemia 1. Both conditions require specific treatment: insulin for Type 1 diabetes and likely parathyroidectomy for primary hyperparathyroidism 2, 6.