Measurement of TSH in Plasma and Pituitary of the Rat by a Radioimmunoassay Utilizing Bovine TSH: Effect of Thyroidectomy or Thyroxine Administration on Plasma TSH Levels

Abstract

A method for radioimmunoassay of bovine TSH has been developed which can be used to measure plasma TSH concentration in the rat and mouse and pituitary TSH concentration in the rat, beef, pig, mouse, sheep, rabbit and monkey. Hormonal specificity has been demonstrated by lack of interference in the assay by rat growth hormone and bovine LH. Preparations of rat FSH cross-react slightly in the assay system, an effect attributable to TSH contamination. Rat LH cross-reacts to a greater degree. The effect is due in part to TSH contamination but may also be due to true antigenic similarity between TSH and LH. The extent of cross-reactivity between LH and TSH is slight and does not interfere with the TSH specificity of the assay since plasma levels are not elevated in gonadectomized animals, and are not detectable in T4-treated animals. Using bioassayed rat pituitary extract as standard, normal (decapitate) plasma TSH in the male rat, 200–250 g body wt, was 47.3 ±7.2, 42.6 ±5.0 and 29.0 ±4.8 mU/100 ml in separate series of animals. Two weeks after thyroidectomy plasma TSH levels had risen to 175.9 ±15.7 mU/100 ml and 129.6 + 17.8 in 2 separate experiments. Treatment of thyroidectomized rats with thyroxine 8 μg/100 g body wt/day suppressed plasma TSH to 13.7 ±3.7 mU/100 ml. In a separate experiment, graded doses of thyroxine given to thyroidectomized rats caused graded inhibition of plasma TSH response, over the range of 0.5 to 4.0 Mg/100 g body wt/day. This technique provides a means of quantitative analysis of pituitary-thyroid feedback control. As inferred from the dose of T4 required to restore TSH levels to the normal, T4 secretory rate in this group of animals was approximately 1.8 μg/100 g body wt/day. (Endocrinology87: 1022, 1970)