A 27 year-old, G1P0 female, presents with severe nausea and vomiting. She is currently at 15 weeks gestation. History is unremarkable for diarrhea, jaundice, and abdominal pain. The patient comments on difficulty in increasing her weight during the pregnancy. She attributes this to the constant nausea and vomiting.

Physical Exam:
Vitals : Heart Rate: 104 BP: 105/60 Temp:36.9 RR: 18
The patient appears uncomfortable but is not in immediate distress.
She has dry mucous membranes and appears pale.
Cardiac exam: normal heart sounds.
Pulmonary exam: unremarkable.
Abdominal exam: no masses, tenderness or guarding.

Investigations:
Hemoglobin: 125 g/L Normal 120-160
White Blood Cell Count: 7.0 x 109/L Normal 4.0 - 11.0 x 109/L
AST: 38 IU/L Normal < 50
ALT: 35 IU/L Normal < 40
ALP: 100IU/L Normal 30 - 140
Bilirubin (total): 16 µmol/L Normal < 20
Lipase: 225 U/L Normal < 300
Creatinine: 85 µmol/L Normal 45 - 125
Sodium: 135 mmol/L Normal 135 - 145
Potassium: 3.5 mmol/L Normal 3.5 - 5.0
Glucose: 5.0 mmol/L Normal 3.5 - 6.5
Urinalysis: negative for protein, glucose, white blood cells and bacteria

The most likely diagnosis is: