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Manoel R. A. Martins, Fabio C. Doin, William R. Komatsu, Turibio L. Barros-Neto, Valdir A. Moises, Julio Abucham, Growth Hormone Replacement Improves Thyroxine Biological Effects: Implications for Management of Central Hypothyroidism, The Journal of Clinical Endocrinology & Metabolism, Volume 92, Issue 11, 1 November 2007, Pages 4144–4153, https://doi.org/10.1210/jc.2007-0941
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Abstract
Context: The biological significance of GH-induced changes in serum TH concentrations is unknown. It has been suggested that serum free T4 (FT4) should be targeted at the high-normal range during GH replacement.
Objective: Our objective was to evaluate the effects of GH replacement on T4 biological effects.
Hypothesis: If GH modulates thyroxine biological effects, serum FT4 should be targeted accordingly.
Design and Setting: We conducted observational (study 1) and interventional (studies 2 and 3)/outpatient studies.
Patients: Thirty-two GH-deficient patients (13 off GH; 22 on l-T4) participated in the study.
Interventions: In study 2, levothyroxine was administered to increase FT4 (>1.0 ng/dl). In study 3, GH was administered or withdrawn.
Main Outcome Measures: We measured FT4, total T3 (TT3), myocardial isovolumic contraction time (ICT), and resting energy expenditure (REE).
Results: In study 1, off-GH and on-GH groups had similar FT4, but off GH showed lower TT3 (P < 0.01) and REE (P = 0.02), higher ICT (P < 0.05) than on-GH and controls. On GH, ICT and REE correlated only with TT3 (r = −0.48; r = 0.58; P < 0.05). Off GH, ICT correlated only with FT4 (P < 0.01). In study 2, off GH, levothyroxine intervention increased FT4 (P = 0.005) and TT3 (P = 0.012), decreased ICT (P = 0.006), and increased REE (P = 0.013); ICT and FT4 changes correlated (r = −0.72; P = 0.06). On GH, levothyroxine increased FT4 (P = 0.0002), TT3 (P = 0.014), and REE (P = 0.10) and decreased ICT (P = 0.049); REE and TT3 changes correlated (r = 0.60; P = 0.05). In study 3, GH decreased FT4, increased TT3, decreased ICT, and increased REE (P < 0.05). REE correlated (P < 0.05) with IGF-I (r = 0.57) and TT3 (r = 0.64). ICT correlated only with TT3 (r = −0.46).
Conclusions: GH replacement improves the biological effects of T4. Serum FT4 should be targeted at the high-normal range in GH-deficient patients only off GH replacement.
