Dickens is describing a scene in the Swiss Alps, and mentions "an idiot sunning his goiter". From the location, the youth was a cretin, a victim of iodine deficiency in utero, because the tiny amounts of iodine needed have been leached out of Alpine soils long ago.
From the Internet:
Endemic goiter and cretinism associated with iodine deficiency have been depicted in paintings and statues since earliest times (55). Only during the past decade, however, have iodine deficiency disorders (IDD) been recognized as the leading cause worldwide of intellectual impairment (36, 69). Development of the central nervous system for normal intellectual functioning depends on an adequate supply of thyroid hormones which require iodine for biosynthesis (21).

Endemic cretinism is the most severe manifestation of the lack of maternal and fetal thyroid hormone arising from severe dietary iodine deficiency (63).

Hallmarks include mental retardation, pyramidal neurological signs in an upper limb distribution, extrapyramidal signs, and a characteristic gait related to the neurological disorder, as well as joint laxity and deformity.