Because Armour thyroid comes from pig thyroids, some endocrinologists feel that there is high pill to pill variability, but this is unlikely to be true.
A recent study published in New England Journal of Medicine in 1999 suggested that brain T4 to T3 conversion may be impaired in some patients and that a select group of patients should be treated with both T4 and T3. Other studies published in Journal of Clinical Endocrinology and Metabolism in 2003 suggested that addition of T3 to T4 treatment is not needed for most patients with primary hypothyroidism.
I recommend that most patients be started on a T4 preparation, which improves symptoms in the large majority of the patients. I have found that most patients prefer Levoxyl or Unithroid to Synthroid, but this varies with each patient. After initial treatment with T4, I adjust their T4 dose until their TSH is between 0.5 and 2 mU/mL. If they remain symptomatic, despite an optimized TSH, then low doses of T3 given two or three times a day can be added cautiously to T4. If patients start with a low blood free T3 level, then I am more inclined to treat them with T4 plus T3. On T4 plus T3 therapy, I use blood tests to make sure the free T4 and free T3 are in the upper-normal range. The TSH value is usually suppressed on combination treatment.
A percentage of patients will have symptomatic improvement on T4 plus T3 therapy. For those that do not improve, I occasionally recommend treatment with dessicated thyroid preparations, usually Armour, plus synthetic T4. This combination is needed, as desicatted thyroid preparations
have a higher T3/T4 ratio than desirable and need to be supplemented with synthetic T4 to achieve
normal ranges of both hormones. Again, I aim for a free T4 and free T3 in the upper-normal range.
Patients with central hypothyroidism can be treated with any of the preparations available for patients with primary hypothyroidism. The difference is that treatment needs to be monitored by aiming for a free T4 and free T3 in the upper-normal range, as TSH is suppressed with proper treatment.