Subclinical
hyperthyroidism (SHyper) is defined as normal levels of free
thyroxine (fT4) and free
triiodothyronine (fT3) with suppressed levels of TSH. Previous studies have reported the individual pathophysiology of endogenous SHyper patients and athyreotic patients receiving TSH suppression
therapy with
levothyroxine; however, apparently no studies have compared the two conditions. Five-hundred-forty untreated endogenous SHyper patients and 1,024 patients receiving TSH suppression
therapy who underwent total
thyroidectomy for
papillary thyroid carcinoma were sampled.
Thyroid hormone profiles and peripheral indices related to
thyrotoxicosis were investigated in endogenous SHyper patients, athyreotic patients receiving TSH suppression
therapy, and healthy participants. Endogenous SHyper patients showed significantly higher
thyroid hormone levels (fT4 [p < 0.001] and fT3 [p < 0.001]), and peripheral indices showed a significant tendency towards
thyrotoxicosis (strong TSH suppression:
alkaline phosphatase [ALP, p < 0.001],
creatinine [Cre, p < 0.001], pulse rate [p < 0.05]; and mild TSH suppression: Cre [p < 0.05]) than healthy participants. In contrast, athyreotic patients receiving TSH suppression
therapy showed a significant tendency towards
thyrotoxicosis than healthy participants only when TSH was strongly suppressed (fT3 [p < 0.001] and Cre [p < 0.001]). Endogenous SHyper patients showed significantly higher fT3 levels (p < 0.001) than athyreotic patients receiving TSH suppression
therapy; however, there was a significant tendency towards
thyrotoxicosis only when TSH was strongly suppressed (ALP [p < 0.05] and pulse rate [p < 0.05]). The effects of endogenous SHyper and TSH suppression
therapy on target organ function are different. Although the serum
thyroid hormone profile is similar to that of the thyrotoxic state, athyreotic patients receiving TSH suppression
therapy with mildly suppressed serum TSH levels are not thyrotoxic.