I have a question about Thyroid hormone. I read that it acts like a lipid. So does that mean that it is like a steroid in terms of it traversing the plasma membrane and then being transported into the nucleus and acting directly on DNA?

 

in addition, people say that steroids are long lasting and this is due to them acting directly on DNA. Are they long lasting because you cannot degrade them or turn them off inside of the nucleus? Then how do we control their level of expression by just creating inhibitors to degrade extra protein that they produce or via RNAi?

Some hormones bind to proteins that bind to DNA and stimulate or inhibit transcription. That is generally true of steroid hormones and also of thyroid hormones. I am not sure of their routes of degradation.

As BabcockHall mentioned, the regulation is enacted via proteins, not directly via the steroids/hormones (most notably proteins of the nuclear receptor superfamiliy). There are therefore multiple layers that modulate the final expression. One is on the level of the protein with all the possibilities such as by abundance, inhibition, activation, etc. On the metabolite level inactivation of thyroid hormones (I assume we are talking T3/T4 here) there are several pathways but generally is a form of deiodination.