Welcome to our forum and sorry to hear of your possible thyroid problems.
I would definately recommend seeing your GP for blood work.
Also ask to be tested for Vit D, Vit B12, folate and ferritin as deficiencies in these are common in people with thyroid issues.
If there is a thyroid problem and it did not reverse itself, the sooner you get it checked the better. Low thyroid hormones may not produce classic symptoms straight away as your body will compensate for a long time but issues such as the weight gain and lethargy could be clues.
Tofu is made by coagulating soy milk which is controversial re thyroid issues,
with many saying there is not a problem as there is very little available data. However, what is generally recognised is that soy may inhibit thyroid medication and so I believe thyroid hormone conversion.
There is many articles, papers, and discussion on soya discussing the merits and down falls but I consider soy to have quite a powerful effect on the body. Isoflavones that mimic oestrogen can be helpful in stabilising menopause systems but now we hear can cause possible cancer risks. As all endocrine glands are linked it would make sense that ALL could suffer the positive / negative effects of soy.
If you want to take extra protein you could try whey powder in smoothies. I have this because my body seems to react positively to protein and I am not a huge meat eater. You can also use a pea, hemp, sunflower or brown rice protein powder but whey contains most. Moderation is key in everything.
However if you are going to eat soy ... it is best to limit amounts and only consume organic and fermented soy.
As most soy is genetically modified, I just completely avoid but small amounts of gluten free tamari, miso, natto, tempeh should be fine.
I think the best diet is a balanced diet with fresh carbs, proteins, fats, veg & fruit.
Your weight gain ...[.. a ridiculous 18 lbs. in a week ...] .... will most likely be fluid retention down to a sluggish lymphatic system which should recover quickly when thyroid hormones become balanced ..(. If they are the problem..)..
With regard to reversing your condition... I don't know but would guess that if it has been recently bought on by an excess of soy ... then by removing the soy, it must give it good chances. Woman using Isoflavones for menopause symptoms notice changes when adjusting dosages indicating the endocrine glands will respond negatively or positively to dose change...... but this is a guess.
I hope you feel better soon,
Studies Showing Adverse Effects of Isoflavones, 1950-2013
Link detailing study of soy & thyroid function. . ..fairly inconclusive but evidencing change in those with iodine deficiency.