A decrease in TSH and T4 levels is exactly what one would expect in a patient receiving exogenous T3 therapy. In fact, the resetting phenomenon (p48) is often not seen unless there is a decrease in T4 levels for a time. It is not alarming if their T4 levels and TSH levels are suppressed even to essentially zero, for a time (provided the patients are feeling very well without any side effects, and their temperatures are not above 98.6). Such low levels are not always necessary, but decreased T4 and TSH levels are an indication that T4 and RT3 levels are being well depleted (which is often the point of T3 therapy).
Hyperthyroidism often causes very low TSH levels and symptoms of hyperthyroidism. Yet proper T3 therapy often causes very low TSH levels without symptoms of hyperthyroidism while providing normal temperatures and a decidedly euthyroid clinical state. This shows that very low TSH levels do not necessarily equate with a hyperthyroid state. Also, there is no evidence that low TSH levels alone necessarily equate with increased osteoporosis. Some doctors have concluded that very suppressive doses of T4 given over many years without regard to patients’ clinical status or body temperatures is associated with increased bone loss. But that doesn’t mean it can be concluded that suppressive doses of T3 given over a period of months to produce normal body temperatures and a clinically euthyroid state also cause increased bone loss. There is no evidence that endogenous hyperthyroidism has ever been generated in a euthyroid patient with suppressive exogenous thyroid medicine. And there is no evidence that endogenous hypothyroidism has ever been generated in a euthyroid patient with exogenous thyroid medicine.