Size heterogeneity of immunoreactive prolactin in hyperprolactinaemic serum.

Thirty-five samples of serum with levels of immunoreactive prolactin greater than 500 mu/l have been fractionated by gel filtration. In samples from seven normal subjects with elevated prolactin levels following TRH or insulin stimulation and samples from twelve patients with prolactin levels greater than 1500 mu/l, over 8% of the immunoreactive material eluted in the position of native or 'little' prolactin. In eleven out of sixteen samples containing 500 to 1500 mu/l the proportion of 'little' prolactin was less than 80% and more than 20% eluted as 'big-big' prolactin. It appears therefore that patients with moderately elevated levels of immunoreactive prolactin may not have genuine hyperprolactinaemia.