Benefit of combined triiodothyronine (LT(3)) and thyroxine (LT(4)) treatment in athyreotic patients unresponsive to LT(4) alone.

Despite some reports, the usefulness of levothyroxine (LT(4)) and levotriiodothyronine (LT(3)) combination therapy in hypothyroidism remains controversial. The objective of this paper is to study a benefit of additional LT(3) in athyreotic patients who failed to normalize TSH on LT(4) alone even with hyperthyroid serum T(4) values. In a survey of 200 athyreotic patients treated between 2006 and 2009, about 7% failed to normalize serum TSH levels following treatment with LT(4), though serum T(4) values in the hyperthyroid range were achieved. These patients (characterized by serum T(4)≥160 nmol/L and TSH≥5.0 mIU/L), were additionally treated with 10 μg b. i. d LT(3). LT(3) and LT(4) combination therapy resulted in decreased serum TSH levels into the normal range (12.8 vs. 1.22 mIU/L; p<0.01) and reduced LT(4) dose (153.3 vs. 117.5 μg; p<0.01) required for normalization of serum T(4) values (170.6 vs. 123.3 nmol/L; p<0.01). Serum T(3) values were higher (1.3 vs. 2.26 nmol/L; p<0.01) than those during monotherapy with LT(4). Our results indicate a subpopulation of athyreotic patients that could significantly benefit from combined LT(4) + LT(3) therapy in restoring normal TSH and thyroid hormone patterns. Further research should be undertaken to provide a genetic basis for these findings.