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Background: Subclinical hypothyroidism is common in older individuals, but the physiologic responses to treatment with levothyroxine (LT4) and liothyronine (LT3) are not well defined in this age group. Methods: We conducted a randomized, double-blind, cross-over study of LT4 and LT3 treatment in men and women aged 70 years and over without anti-thyroid peroxidase antibodies with persistent subclinical hypothyroidism, defined as having a TSH level between 4.5 and 19.9 µIU/mL with a normal free thyroxine (FT4) level at two consecutive time points. Physiologic outcome measures assessed after achieving a TSH level of 0.5-1.5 µIU/mL with each therapy included vital signs, weight and body composition, bone mineral content and bone density, lipids, resting energy expenditure (REE), cognitive function, quality of life, and thyroid symptoms. Results: Thirteen participants [mean (SD) age 77 (5) years], 4 women and 9 men, completed the study. Baseline mean TSH was 4.84 (1.29) µIU/mL. The mean LT4 dose was 105 (36) µg/day [1.4 (0.5) µg/kg/day] and LT4 dose was 34 (9) µg/day [0.4 (0.1) µg/kg/day]. Mean time on LT4 was 200 days and on LT3 was 231 days, with a 28 day washout period. Compared with baseline, participants had an average weight loss of 1.1 kg on LT4 (p&amp;lt;0.02) and 2.5 kg on LT3 (p&amp;lt;0.001), which was significantly different between the two treatments (p=0.01). Fat mass decreased by an average of 0.7 kg on LT4 (p=0.03 vs. baseline) and 1.5 kg on LT3 (p&amp;lt;0.01 vs. baseline) and differed between treatments (p=0.01). There was a significant difference in total cholesterol of 13.3 mg/dL (p&amp;lt;0.001) and in low-density lipoprotein cholesterol (LDL) of 10.8 mg/dL (p&amp;lt;0.001) between LT4 and LT3 treatment arms; for both, the levels were lower on LT3 than LT4. No differences were seen in the other assessed outcomes. Conclusions: In a cross-over study of treatment of LT4 or LT3 in persistent subclinical hypothyroidism, participants lost fat mass and weight after each treatment, with a greater decrease after treatment with LT3. These findings support different physiologic responses to LT4 compared with LT3.

journal of the endocrine society

Physiologic Effects of Levothyroxine and Liothyronine in the in Older Individuals With Persistent Subclinical Hypothyroidism: A Randomized, Double-Blind, Cross-Over Study