I bring this up because I hear this often from my clients who are taking thyroid meds and I thought it was worth sharing a naturopathic perspective as to why this may be with you.
1. Sometimes I find this is due to a problem/sluggishness in the conversion pathway of T4 (which is the hormone synthroid is made of) to T3 (the more active thyroid hormone that is responsible for energy, metabolism, mood, etc.). This pathway needs several specific nutrients such as Selenium, Iodine and Iron to run smoothly and too much Cortisol (stress hormone) puts a wrench in it. There are formulas such as Thorne's Thyrocsin which provides the raw materials for healthy thyroid function so if the nutrition aspect is lacking I recommend this.
2. Autoimmune thyroid - in the case of Hashimoto hypothyroidism which is fairly common and where there is an autoimmune component, the low energy can be in part due to an underlying digestive weakness/some inflammatory process in the gut that has not been resolved yet. So determining food sensitivities and overall digestive health can be pivotal in helping you feel better.
3. Last but not least the effect of stress on the thyroid gland and whole body can be mitigated by lifestyle and therapeutic work that help build resiliency to stress through supporting the adrenal glands which work closely with the thyroid gland. The glands work together to restore and maintain health in your body so when you give one support(thyroid) without the other(adrenals) the overall effect on well-being can be less than expected - which is what I hear from my clients. "My thyroid bloodwork is within range now but I still don't feel great." Thyroid issues in my experience are often due in part to low functioning adrenals from ongoing stress/inflammation and so addressing the adrenals is key.
Here's how I address this: Along with adrenal fine tuning with nutrition and herbs, I find a series of 6 acupuncture with gentle tuning fork activation treatments that take about 30 minutes each are a good place to start for specifically nourishing the adrenals and building calm and energy in the body and mind. Simple mindfulness exercises to practice at home increase any adrenal nourishing protocol especially when combined with knowledge of adrenal hormone rhythms.
FYI - If you are on thyroid meds remember to account for the extra calcium needed when taking these. Also, another nutrient my clients on thyroid meds seem to benefit from is B12. If they have the symptoms of low B12 I do a test run to see if B12 will be of benefit - a B12 shot can be a pretty seamless way to make a big impact to energy levels and mood.