What are the cortisol results and reference range? And what time of day were you tested? Were you also tested for Free T3, Vitamin D, B12 and ferritin?
Your doctor is following the decades old belief about TSH being the most important indicator of thyroid status. That is incorrect. TSH is supposedly an indicator of thyroid hormone levels. yet data clearly show that TSH has only a weak correlation with important free thyroid hormone levels and has a negligible correlation with a person's Tissue T3 Effect, which determines a person's thyroid status and and related symptoms. TSH is useful as a diagnostic for thyroid status only when at extreme levels and only in the untreated state.
There are scientific studies showing that when a hypo patient is taking thyroid mediation adequate to relieve hypo symptoms, the TSH is usually suppressed below range. Why would that indicate hypothyroidism when the Free T4 and Free T3 levels are well within range? Our bodies evolved with low flow of thyroid hormone throughout the day and night. In the treated state, when taking a thyroid med only once or twice daily it totally changes the equilibrium among TSH, Free T4 and Free T3. This subject was the important conclusion from the recent, excellent scientific study I mentioned above that concluded, "Hypothyroid symptom relief was associated with both a T4 dose giving TSH-suppression below the lower reference limit and FT3 elevated further into the upper half of its reference range." I am sending a PM with additional info on that. To access, just click on your name and then from your personal page click on messages.
So your doctor is correct in reducing your T4 med, since it does not need to be more than mid-range; however, he is wrong not to test for Free T3 and add a source of T3 to your med to raise your Free T3 level above the middle of its range. In the PM I will provide information that will hopefully give you all the evidence needed to get your doctor to reconsider and do the additional tests required so that you can supplement as needed to optimize, and also prescribe T3 med adequate to raise your Free T3 above mid-range. After that, if the doctor resists doing all this then you will need to find a good thyroid doctor that will diagnose and treat clinically rather than just based on biochemical tests that don't accurately reflect your thyroid status.
The correct definition of hypothyroidism is "insufficient T3 effect in tissue throughout the body due to inadequate supply of, or response to, thyroid hormone." Hypothyroid patients taking T4 thyroid medication like Levothyroxine frequently find that their body does not adequately convert the T4 to usable T3. A recent excellent scientific paper concluded that: " Hypothyroid symptom relief was associated with both a T4 dose giving TSH-suppression below the lower reference limit and FT3 elevated further into the upper half of its reference range." Low metabolism is a common symptom of hypothyroidism. So it is likely that your weight issue and other symptoms are due to inadequate Free T3 level, even though your Free T4 was above range.
In addition the response to thyroid hormone, mentioned in the definition, is affected by a number of variables such as Reverse T3, cortisol, Vitamin D, B12 and ferritin. So I recommend that you get those tested, along with Free T3 and Free T4. Then you will be able to confirm to the doctor that you need to add a source of T3 to your med, while reducing your T4 dosage. In addition you need to know if you have excess Reverse T3 or low/high cortisol. For the others you should supplement as needed to optimize. D should be at least 50 ng/mL, B12 in the upper part of its range, and ferritin should be at least 100.
If you can get these done, then please post results and reference ranges here and we will be glad to help interpret and advise further.
If you want to confirm what I recommend please click on my name and then scroll down to my Journal and read at least the one page overview of a full paper on Diagnosis and Treatment of Hypothyroidism: A Patient's Perspective.
Hypothyroidism is correctly defined as "insufficient T3 effect in tissue throughout the body due to inadequate supply of, or response to, thyroid hormone". So there are numerous variables that could have affected you and caused you to start having hypothyroid symptoms again.
There is no biochemical test that can measure Tissue T3 Effect, so there is no direct measure for it, so indirect measures are required. The best of those is an evaluation for symptoms that occur more frequently with hypothyroidism. You have mentioned one of those (weight gain). Do you have any other symptoms?
The other indirect measures for Tissue T3 Effect are Free T4, Free T3, Reverse T3, cortisol, Vitamin D, B12 and ferritin. Free T4 and Free T3 provide a measure of the biologically active serum thyroid hormone levels. Reverse T3, cortisol (along with a Free T3 from same blood draw), Vitamin D, B12 and ferritin significantly affect the response to thyroid hormone at the cellular level. So, if not tested for all these you need to do so.
There is more to discuss, but first please tell us about any other symptoms you have and also what thyroid medication and dosage you are taking. Also, please confirm that T4 test was Free T4, and post the reference range.