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Medullary thyroid carcinoma in Norway.

Fifty-four primary cases of medullary carcinoma of the thyroid (MCT) traced in Norway, 1960–74, were studied. The mean annual incidence rate was 0.1 per 100,000; the frequency of the tumour among all thyroid carcinomas registered in Norway was 3.4 per cent. The study of the geographical distribution of cases revealed a significantly higher proportion in rural than in urban areas, and in coastal as compared with inland counties. There was also an unexpected accumulation of cases in a few sparsely inhabited valleys in the southern and western part of the country. The familial occurrence of the disease was investigated by means of the family history, pedigree studies and measurement of immuno-reactive calcitonin (iCT) in serum of relatives. The family history was found to be of limited value in discriminating between familial and sporadic cases. Family screening with serum calcitonin measurements revealed hypercalcitoninemia in relatives of 4 probands, 2 of which were apparently sporadic. Through pedigree studies it became evident that 15 probands were distantly releated to one or two of the others, and the study of grandparents' origin revealed that 16 additional probands had one or more grandparents originating in the same rural municipality as the grandparents of at least one of the others. The high degree of inbreeding in these areas makes it possible that some, if not all, of these 16 probands in fact are genetically related. The pattern of inheritance was in one, possibly in two kindreds, consistent with autosomal dominance. In three other kindreds an autosomal recessive trait is discussed. The familial and also the geographical relationships demonstrated in the other probands may also be an expression of genetic etiology. The nature of this is not clear, but it does not fit in with simple Mendelian inheritance.