Yes, fluid weight can be lost via sweating.
The bigger question is what is causing the night sweats. This should certainly be mentioned to your physicians.
Chronic disease, infection (i.e. TB, HIV, endocarditis etc.), lymph node disease (cancer), rheumatological disease, medications, thyroid disorders are amongst the possible causes.
From UptoDate:
"A chest X-ray and PPD are appropriate for all such patients as an initial evaluation for possible tuberculosis, in addition to a complete blood count, TSH, and blood cultures. If these are normal, and drenching night sweats persist, clinicians should obtain a torso computed tomography (CT) scan to evaluate for lymphoma, solid tumor, or abscess. If no diagnosis is apparent and severe night sweats persist, additional evaluation may include HIV serology and a bone marrow biopsy."
These options can be discussed with your personal physician.
Followup with your personal physician is essential.
This answer is not intended as and does not substitute for medical advice - the information presented is for patient education only. Please see your personal physician for further evaluation of your individual case.
Kevin, M.D.
kevinmd_b
Reference:
Smetana. "Approach to the patient with night sweats". UptoDate, 2006.