Can the underactive thyroid cause weight loss or only weight gain?

Hypothyroidism : Hypothyroidism is usually associated with weight gain more than weight loss.

Weight Gain: Hypothyroidism (low) is associated with gaining weight, because your metabolism is too slow and sluggish. The most common cause is autoimmune, called Hashimoto's. Other symptoms include fatigue/lethargy, muscle aches, menstrual irregularity, constipation, feeling cold, dry skin, depression, and coarse hair. you may have a goiter/enlarged thyroid. Blood tests can show anemia, high cholesterol.

Not typical: It would be very atypical to loss weight from untreated hypothyroidism, unless your appetite were significantly compromised.