Most Likely Diagnosis: Type 1 Diabetes Mellitus (Option A)
This patient has Type 1 diabetes mellitus with concurrent primary hyperparathyroidism—two separate coexisting conditions that require distinct management approaches. 1
Diagnostic Reasoning
Type 1 Diabetes is Confirmed by Multiple Features:
Classic symptoms of polyuria and polydipsia indicate hyperglycemia-induced osmotic diuresis, which is characteristic of diabetes mellitus rather than other causes of polyuria 2, 3
HbA1c of 8.5% confirms significant hyperglycemia and meets diagnostic criteria for diabetes 1
C-peptide of 0.09 is critically low and confirms absolute insulin deficiency due to beta-cell destruction, which is the hallmark of Type 1 diabetes 1, 3
Positive family history of Type 1 DM increases the likelihood of autoimmune diabetes, as Type 1 diabetes has genetic predisposition 1
Why Not the Other Options:
Type 2 DM (Option B) is excluded because Type 2 diabetes presents with preserved or elevated C-peptide levels due to insulin resistance, not the profound C-peptide deficiency seen here (0.09) 2
Central diabetes insipidus (Option C) is excluded because the presence of elevated HbA1c and low C-peptide indicates diabetes mellitus, not diabetes insipidus. Additionally, polyphagia would not occur with diabetes insipidus 3, 4, 5
Hyperparathyroidism (Option D) is present but is not the primary diagnosis explaining the polyuria and polydipsia. While hyperparathyroidism can cause mild polyuria through hypercalcemia-induced nephrogenic diabetes insipidus, it does not explain the severe hyperglycemia (HbA1c 8.5%) or profound insulin deficiency (C-peptide 0.09) 6
Clinical Significance of Concurrent Conditions
The coexistence of Type 1 diabetes and primary hyperparathyroidism is clinically important because elevated intracellular calcium from hyperparathyroidism can worsen glucose control through insulin resistance 1
Parathyroidectomy may improve glucose control in 37-77% of diabetic patients with concurrent hyperparathyroidism, making early recognition of both conditions essential 1
The diagnosis of hyperparathyroidism is supported by elevated PTH and calcium levels, even though PTH levels may appear "inappropriately normal" in the setting of hypercalcemia 6
Immediate Management Required
Insulin therapy must be initiated immediately because this patient has Type 1 diabetes with marked hyperglycemia 1, 3
Start basal insulin at 0.5 units/kg/day administered once daily, typically at bedtime 3
Initiate prandial rapid-acting insulin at 4-6 units before each main meal 3
Check urine ketones immediately to assess for diabetic ketoacidosis, as patients with Type 1 diabetes presenting with polyuria, polydipsia, and HbA1c >8% are at high risk 2, 3
Evaluate hyperparathyroidism for surgical candidacy once diabetes is stabilized, as corrected calcium >1 mg/dL above upper limit of normal is a surgical indication 1
Common Pitfall to Avoid
Do not delay insulin initiation while pursuing workup for hyperparathyroidism or other causes of polyuria. The combination of classic symptoms, elevated HbA1c, and profoundly low C-peptide mandates immediate insulin therapy to prevent diabetic ketoacidosis 3