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Basal Oxygen Uptake: A New Technique for an Old Test^*

Divisions of Nephrology and Pulmonary Medicine, Department of Internal Medicine, Veternas Administration Medical Center and the University of Iowa Colleage of Medicine Iowa City, Iowa 52242

Address all correspondence and requests for reprints to: Victoria S. Lim, M.D., W336-16 GH, Division of Nephrology, Department of Internal Medicine, University Hospitals and Clinics, Iowa City, Iowa 52242.

To assess the metabolic effects of T₄ and T₃, we measured serum total T₄ (TT₄), free T₄ (FT₄), total T₃ (TT₃), TSH, and basal oxygen uptake (VO₂) in eight normal subjects in the basal state and after treatment with L-T₃ (T₃) and sodium ipodate for 2 weeks. T₃ treatment resulted in a rise of serum TT₃ from a baseline of 137 ± 16 (±SE) to a peak of 239 ± 15 ng/dl. Serum TT₄ declined from 8.14 ± 0.56 to 6.08 ± 0.43 µg/dl, FT₄ from 1.59 ± 0.13 to 1.03 ± 0.05 ng/dl, and TSH from 1.74 ± 0.24 to 0.56 ± 0.16 /µµU/ml. Basal VO₂ increased from 2.66 ± 0.11 to 3.15 ± 0.09 ml/kg·min. Ipodate, on the other hand, led to a lower serum TT₃ concentration (102 ± 21 ng/dl), higher serum TT₄ and FT₄ (9.59 ± 0.5 µg/dl and 1.91 ± 0.13 ng/dl, respectively), and elevated TSH (3.64 ± 0.14 µU/ml). Basal VO₂ was reduced to 2.44 ± 0.06 ml/kg · min. Linear regression analysis revealed an excellent positive correlation between serum TT₃ and basal VO₂ (n = 25; r = 0.747; P < 0.001) and a significant negative correlation between serum TT₃ and TSH (n = 26; r =–0.526; P <0.01). Serum TT₄ and FT₄ correlated negatively with VO₂ and positively with serum TSH. The higher T₄ level during ipodate treatment was associated with lower VO₂ and higher TSH, and vice versa when T₄ was suppressed while receiving T₃. When ipodate was given concomitantly with T₃ to five subjects, only the effects of T₃,characterized by increased VO₂ and decreased TSH, were evident. These data indicate that both basal VO₂ and serum TSH are sensitiveindices of thyroid hormone activities. The latter gives only the directional change (hyper- or hypothyroidism), while the former more accurately quantitates the magnitude of the derangement. Moreover, it appears that in man, T₃, and not T₄, is the primary hormone thatregulates thermogenesis and TSH secretion.

^* This work was supported by a V.A. Merit Review Award (to V.S.L.) and Grant RR-59 from the GCRC, Division of Research Resources, NIH.

Received August 5, 1985.

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