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Growth Hormone Replacement Improves Thyroxine Biological Effects: Implications for Management of Central Hypothyroidism

Divisions of Endocrinology (M.R.A.M., W.R.K., J.A.) and Cardiology (F.C.D., V.A.M.) and Centro de Medicina do Esporte (T.L.B.-N.) da Escola Paulista de Medicina-Universidade Federal de São Paulo, São Paulo, Brazil 04039

Address all correspondence and requests for reprints to: Julio Abucham, M.D., Ph.D., Division of Endocrinology, Escola Paulista de Medicina-Universidade Federal de São Paulo, Rua Pedro de Toledo 910, São Paulo, Brazil 04039-002. E-mail: julioabucham{at}uol.com.br.

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 In study 3, GH was administered or withdrawn.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.