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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 In study 3, GH was administered or withdrawn.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 …