Some antidepressant medications can cause weight gain, especially the older tricyclic antidepressants (TCAs) such as Tryptizol/Saroten (amitriptyline), and Anafranil (clomipramine); as well as newer drugs such as Remeron (mirtazapine). Lithium (for manic-depressive disorder) often causes weight gain. The most common antidepressants known as SSRI’s, for example, Celexa (citalopram) and Zoloft (sertraline) do not appear to impact weight significantly. More on depression
Your doctor can easily arrange for you to take a blood test to measure the concentration of thyroid stimulating hormone (TSH). If the test comes back and everything looks good, your thyroid gland is probably fine. For a more exact diagnosis, you can ask them to measure the actual levels of thyroid hormones in the blood (T3 and T4), as sometimes these are low even if the TSH is within the fairly broad normal range.
Aude, Y., A. S, Agatston, F. Lopez-Jimenez, et al. “The National Cholesterol Education Program Diet vs a Diet Lower in Carbohydrates and Higher in Protein and Monounsaturated Fat: A Randomized Trial.” JAMA Internal Medicine 164, no. 19 (2004): 2141–46. doi: 10.1001/archinte.164.19.2141. jamanetwork.com/journals/jamainternalmedicine/article-abstract/217514.
Some people eat three times a day and occasionally snack in between (note that frequent snacking could mean that you’d benefit from adding fat to your meals, to increase satiety). However, there’s some evidence that frequent snacking isn’t wise when trying to lose weight. Some people only eat once or twice a day and never snack. Whatever works for you. Just eat when you’re hungry.
Obviously, it’s still possible to lose weight on any diet – just eat fewer calories than you burn, right? The problem with this simplistic advice is that it ignores the elephant in the room: hunger. Most people don’t like to “just eat less”, as it may result in having to go hungry forever. Sooner or later, a normal person will likely give up and eat, hence the prevalence of “yo-yo dieting”. While it should be possible to lose weight on any diet, some appear to make it easier and some to make it much harder.