In 1825, Berthold decided to practise medicine in Berlin and began experimenting on the effects of coal gas and mercury on the body.

Unsettled, he continued to tour Germany and Franc, attending the lectures of other contemporary luminaries Georges Cuvier, Étienne Geoffroy Saint-Hilaire and André Marie Constant Duméril.

He returned to his alma mater as a privatdozent in medicine and began to teach physiology; he spent the rest of his career there.

[3] He collaborated with Robert Bunsen in 1834 to develop the usage of hydrated iron oxide as an antidote for arsenic poisoning.

Berthold taught and encouraged the young Carl Bergmann (anatomist), who became known for experiments on thermoregulation and who coined the terms poikilotherm and homeotherm.

Castrati - men who were castrated before puberty - were kept in harems to ensure females remained chaste; from the Middle Ages boys were castrated to keep youthful-sounding singing voices due to the lack of an Adam's apple, but in adulthood they also kept a straight hairline, grew large chests and unusually long limbs.

[2][4] Despite this ground-breaking result, Berthold abandoned completely any further developmental work in this field and his contemporaries also showed little interest.

The non-reductionist philosophy prevalent amongst physiologists at Göttingen might have coloured their thinking; Rudolph Wagner repeated the experiment without success, and perhaps this also had an effect.

No major progress was made until five decades later with some dramatic successes in the treatment of thyroid-based illnesses by Victor Horsley and, soon after, his student George Murray.