Hello I have gone through your question. Levothyroxine is given when the thyroid does not produce enough of this hormone on its own. Levothyroxine works best if you take it on an empty stomach, at least 30 minutes before breakfast, at the same time each day, with a full glass of water. I don t know what your weight reduction pills contain and i can say that certain medicines can make levothyroxine less effective if taken at the same time. For this reason i suggest to avoid taking your fasting dietary pill within 4 hours before or 4 hours after you take levothyroxine. Weight gain can be a side effect of Levothyroxine.But don t worry! You should keep in mind that temporary fluctuations in body weight may be common over the first few weeks or months of synthroid treatment; these may subside with continued synthroid administration. Other important elements that may affect your weight gain include: synthroid dosage, duration of administration, other medications or drugs, and your lifestyle. I suggest to discuss with your treating endocrinologist about all these elements ,if your have weight gain even after the first weeks of treatement and you suspect that levothyroxine administration is the culpit of your weight changes. Hope this is helpful. If you have other questions,feel free and ask. Best regards.