Dr. Stengler: Are you on thyroid replacement but still feel like you have symptoms of hypothyroidism? Perhaps you've been on thyroid medication for a while and still feel sluggish, tired, constipated, or have joint pain and foggy thinking. You're wondering: why am I still feeling this way? My doctor tells me my blood tests look good, yet I still have these symptoms.
The problem may well be that you have suboptimal or low levels of T3. Most thyroid medications people take — Synthroid, Levoxyl, levothyroxine — are T4, which is the main storage form of thyroid hormone. In terms of physiology, T4's primary function is to be converted into the more active T3, also known as liothyronine.
When monitoring thyroid function, I always look at multiple biomarkers. Most doctors test TSH — thyroid stimulating hormone, the signal from the pituitary gland to the thyroid — and sometimes T4 levels. Unfortunately, most doctors are not measuring total T3 and free T3, which is the most biologically active thyroid hormone.
There are several reasons T3 levels can be low. Nutritional deficiencies in iodine, selenium, zinc, or copper — all involved in thyroid metabolism — can impair T3 production. Liver or kidney problems can prevent the conversion of T4 to T3. High stress and certain pharmaceutical medications can also block that conversion.
For many patients, I use combination therapy with both T4 and T3, available from regular pharmacies and especially from compounding pharmacies. Research suggests that up to 20% of people on T4-only therapy still don't feel their symptoms are adequately addressed — and in my experience that percentage is actually much higher, because we're not just aiming to eliminate symptoms. We want patients to feel optimal in their energy, cognitive function, and vitality. Thyroid hormones affect essentially every cell in the body, and if T3 levels are suboptimal, optimal health simply isn't possible.
For T3 therapy specifically, you can use the standard form called Cytomel from regular pharmacies, but in my opinion the better option for most patients is sustained-release T3 from a compounding pharmacy. Regular T3 has most of its biological effect within about three to four hours, whereas sustained-release T3 remains active throughout the day. I often have patients take it twice daily for optimal levels from morning to evening — supporting metabolism, weight management, and energy, and helping those prone to water retention or daytime fatigue.
This is worth discussing with your doctor: whether you might be a candidate for T3 therapy as part of your hypothyroidism treatment. I have many videos on hypothyroidism and hormone replacement on my YouTube channel — check those out for more information.
Still feel tired, sluggish, or foggy even though your thyroid labs look “normal”? In this video, Dr. Mark Stengler reveals why many people continue to experience hypothyroid symptoms despite taking medication. He explains the critical difference between T4 and T3, why most doctors don’t test or treat properly for low T3, and how functional medicine approaches can help restore true thyroid balance. If you’ve been frustrated by ongoing symptoms, this is a must-watch.
