Yes, breathlessness can be due to being undermedicated.
I will post a link below re the use of T3 and a couple of excerpts from it:-
(please note the Metabolic Treatment of Fibromyalgia is out of print):-
"If you don't have a copy of The Metabolic Treatment of Fibromyalgia, I recommend it. In the book, I describe and explain the function of thyroid hormone receptors. I also provide massive documentation for the safety and effectiveness of T3 and T4/T3 therapies as alternatives to the generally ineffective T4-replacement. You may find the documentation of value if you have occasion to defend you're practice of T4/T3 therapy.
You asked, if we don't have T4 receptors, "then why do we need T4 supplementation rather than just T3 alone?" With rare exception, we don't.
No one can rationally defend T4 supplementation on scientific grounds. I say this because the widespread use of T4 supplementation is not based on scientific studies that show it to be safer or more effective than the use of T3 alone. Instead, its widespread use is the result of a financial venture between the endocrinology specialty and corporations that profit from sales of the most commonly prescribed brands of T4".
"And finally, why do I specify that the typical patient use one full dose of non-timed-release Cytomel for life? Because extensive testing has shown that this is safe, effective, and most economical—when used within the context of our entire protocol."
Lab studies have shown that mutant, low affinity T3-receptors properly regulate gene transcription only after they're exposed to saturation amounts of T3. Under normal conditions, T3-receptors aren’t saturated, and the use of sustained-release T3 isn’t likely to provide saturation. Saturation isn’t likely to occur with sustained-release T3 because the T3 enters cells only in small amounts over an extended time. As a result, it’s not likely that large enough amounts of T3 will reach the receptors at one time to saturate them. But plain T3 in single doses enters cells in larger amounts. These larger amounts may provide the saturation needed to provoke transcription regulation by mutant T3-receptors.
I have taken T4/T3 and T3 alone in one dose and have had no side effects.