A lot of things can interfere with conversion and T3 signalling, including some aspects of the diet. I wouldn’t recommend taking T3 without lab testing and preferably help from a doctor.
I’d say research it more, but there’s a lot of crazy junk online telling you to take 100mg+ per day along with hydrocortisone and iron supps! Keep in mind that your body produces ~3mg/hour and it’s used up relatively fast. Taking much above that suppresses your thyroid itself. Same with HC suppressing the adrenals and iron is just toxic in anything above absolutely necessary levels.
You might want to look into selenium deficiency, cutting out stress and cardio (stress), and avoiding oestrogens and bacterial toxins and maintaining glycogen stores to support your liver, where 70% of conversion takes place. Stable blood sugar and circadian rhythms are also vital.