While it’s important to have adequate levels of both FT4 and FT3, beware of FT3 rising too high for your body’s needs when you're on replacement hormone. Most people do best with an FT3 level at or slightly above the middle of the reference range and an FT4 level closer to the high end of the range.
While elevated FT3 levels have long been associated with obesity, recent studies show that elevations of both FT3 and TSH are associated with increased levels of visceral fat (the belly fat associated with atherosclerosis and other medical concerns).
In addition, several studies have demonstrated an association between an elevated FT3 with insulin resistance and hypertension. With a rising interest in the T3 only replacement hormone protocol, the problems associated with an elevated FT3 are factors to consider. Studies also show that a low FT4 is associated with an increased waist circumference.
Although an elevated FT3 may cause weight gain in Graves' patients who initially have a normal or low FT4, in most cases, the increased metabolism in active Graves' disease offsets this effect or is compensated for by metabolic changes associated with Graves' disease. However, it can still be the problem in newly diagnosed patients reporting increased weight gain.
S.J. Bakker, KC ter Maaten, C Popp-Snijders, et al. "Triiodothyronine: a link between the insulin resistance syndrome and blood pressure? Journal of Hypertension," Journal of Hypertension, Dec 17, 1999 via Pub Med
G. De Pergola, A. Ciampolillo, S. Paolotti, P. Trerotoli, R. Giorgino
"Free triiodothyronine and thyroid stimulating hormone are directly associated with waist circumference, independently of insulin resistance, metabolic parameters and blood pressure in overweight and obese women." Clinical Endocrinology, Article first published online: 15 APR 2007