Specific uptake of human growth hormone by the liver of severely hypothyroid rats.

Severe thyroid hormone deficiency results in marked impairment of body growth. This is due, at least in part, to impaired growth hormone (GH) synthesis. We hae studied the possible effects of severe thyroid hormone deficiency on liver receptors for GH and for prolactin (PRL) by an in vivo technique. Female thyroidectomized (T) rats and age-paired controls (C) were injected iv with tracer amounts of biologically active monoiodinated hGH, alone or together with 200 micrograms/100 g bw of native hGH, bGH or oPRL. The liver uptake of labelled compounds, and the liver to serum radioactivity ratio was measured 20 min later. The liver to serum radioactivity ratio of C rats was decreased both by native bGH (purely somatogenic) and native oPRL (purely lactogenic). That of the T rats could only decrease with bGH. Such results confirm data obtained in vitro indicating that in the severely hypothyroid rat liver there is a marked decrease in lactogenic binding and strongly suggest that specific binding of growth hormone by the liver is not similarly affected.