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Nature of suppressed TSH secretion during undernutrition: Effect of fasting and refeeding on TSH responses to prolonged TRH infusions

  • Kenneth D. Burman
    Correspondence
    Address reprint requests to Kenneth D. Burman, M.D., Kyle Metabolic Unit, Endocrine-Metabolism Service, Ward 47, Walter Reed Army Medical Center, Washington, D.C. 20012.
    Affiliations
    Kyle Metabolic Unit, Endocrine-Metabolism Service, Walter Reed Army Medical Center, Washington, D.C., USA

    the Division of Medicine, Walter Reed Army Institute of Research, Washington, D.C., USA
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  • Robert C. Smallridge
    Affiliations
    Kyle Metabolic Unit, Endocrine-Metabolism Service, Walter Reed Army Medical Center, Washington, D.C., USA

    the Division of Medicine, Walter Reed Army Institute of Research, Washington, D.C., USA
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  • Robert Osburne
    Affiliations
    Kyle Metabolic Unit, Endocrine-Metabolism Service, Walter Reed Army Medical Center, Washington, D.C., USA

    the Division of Medicine, Walter Reed Army Institute of Research, Washington, D.C., USA
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  • Richard C. Dimond
    Affiliations
    Kyle Metabolic Unit, Endocrine-Metabolism Service, Walter Reed Army Medical Center, Washington, D.C., USA

    the Division of Medicine, Walter Reed Army Institute of Research, Washington, D.C., USA
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  • Nancy E. Whorton
    Affiliations
    Kyle Metabolic Unit, Endocrine-Metabolism Service, Walter Reed Army Medical Center, Washington, D.C., USA

    the Division of Medicine, Walter Reed Army Institute of Research, Washington, D.C., USA
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  • Phyllis Kesler
    Affiliations
    Kyle Metabolic Unit, Endocrine-Metabolism Service, Walter Reed Army Medical Center, Washington, D.C., USA

    the Division of Medicine, Walter Reed Army Institute of Research, Washington, D.C., USA
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  • Leonard Wartofsky
    Affiliations
    Kyle Metabolic Unit, Endocrine-Metabolism Service, Walter Reed Army Medical Center, Washington, D.C., USA

    the Division of Medicine, Walter Reed Army Institute of Research, Washington, D.C., USA
    Search for articles by this author
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      Abstract

      TSH responses to 4-hr continuous TRH infusions of approximately 0.8 μg/min were assessed during feeding (1500 Kcal), fasting, and refeeding (1500 Kcal) intervals in 9 euthyroid obese subjects. The total area under the TSH response curve was 1854 ± 322 μU/ml · 4-hr during feeding, decreased to 1359 ± 199 μU/ml · 4-hr (p < 0.01) on the 10th day of fasting, and remained low, being 1405 ± 185 μU/ml · 4-hr, despite refeeding a 1500 Kcal diet (40% carbohydrate, 40% fat, 20% protein) for 5 days. Baseline serum T3 concentrations were 167 ± 11 ng/dl during feeding, 86 ± 8 ng/dl during fasting, and 119 ± 12 ng/dl during refeeding. The observed decreases in TSH release appeared to correlate with decreased biologic action on the thyroid gland since the net rise in T3 during the infusion was less in fasting and refeeding than in the control (fed) period. Basal serum rT3 levels were 42 ± 5 ng/dl during feeding, rose as expected to 56 ± 5 ng/dl during fasting (p < 0.005), and were completely restored to normal during refeeding (36 ± 5 ng/dl). These data suggest that: (1) TSH responsiveness to prolonged TRH infusion is diminished during fasting and does not return to control (fed) values despite 5 days of refeeding a 1500 Kcal diet; (2) net T3 increases observed during the TRH infusion are greater in the fed period than in the fasting or refeeding periods; and (3) 5 days of refeeding a 1500 Kcal diet (40% carbohydrate, 40% fat, 20% protein) did not return the T3 to its original fed value whereas rT3 was completely restored to control values. Lastly, since the TSH response was lower both during the early and late phases of the infusion, the decrease in ΔTSH to a bolus of TRH during fasting appears to represent one manifestation of a more general suppression of TSH neogenesis associated with caloric deprivation.
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