Immunoreactive insulin-like growth factor I (IGF-I) and IGF-I-binding protein content in human thyroid tissue

We measured immunoreactive insulin-like growth factor I (IGF-I) in extracts of normal and nodular thyroid tissue obtained at surgery from patients with nontoxic goiter. The nodular tissues contained a higher concentration [mean, 279.0 +/- 69.7 (+/- SE) mU/g] than paired normal tissues (115.5 +/- 17.9 mU/g; P = 0.024; n = 12); a difference was evident in all but one patient. Sephadex G-50 gel filtration of tissue extracts revealed two immunoreactive peaks, the first in the void volume of the column, and the second in the elution volume of authentic IGF-I. The first peak was identified as IGF-I-binding protein by sodium dodecyl sulfate-polyacrylamide gel electrophoresis after cross-linking with iodinated IGF-I. Isolated thyroid cell membranes contained high affinity IGF-I-binding sites of similar affinity and numbers in both normal and nodular thyroid tissue. The IGF-I content of six thyroid cancer extracts was higher than that of normal thyroid tissue, but the IGF-I content of thyroid tissue from six patients with Graves' disease and five patients with Hashimoto's thyroiditis was similar to that in normal thyroid tissue. These data suggest that the stimulatory effect of TSH on thyroid cell proliferation could be mediated through IGF-I action and suggest that an increase in IGF-I production could sustain the goitrogenic process.