In writing the history of a disease, every philosophical hypothesis whatsoever, that has previously occupied the mind of the author, should lie in abeyance.

" A gentleman of fortune he diagnosed with hypochondria was at length told he could do no more for him but that there was living at Inverness a certain Dr Robertson who had great skill in cases like his; the patient journeyed to Inverness full of hope and finding no doctor of the name there came back to London full of rage but cured withal of his complaint. Among his many achievements was the discovery of a disease Sydenham's Chorea also known as St Vitus Dance. If there were any suspicion that the opposition to him was quite other than political it would be set at rest by the testimony of Dr Andrew Brown who went from Scotland to inquire into Sydenham's practice and has incidentally revealed what was commonly thought of it at the time in his Vindicatory Schedule concerning the New Cure of Fevers.

He became the undisputed master of the English medical world and was known as 'The English Hippocrates’. His brother was Colonel William Sydenham. Thomas fought for the Parliament throughout the English Civil War and at its end resumed his medical studies at Oxford.