Have been looking through some very old papers about thyroid and found one co-authored by Charles Harington, who was very much to the fore in efficient isolation of thyroxine from animal thyroid, and co-inventor of a method of synthesising thyroxine.
The irony from today's perspective lies in this paragraph:
CLXXIX. SOME DERIVATIVES OF THYROXINE.
BY JULIUS NICHOLSON ASHLEY
AND CHARLES ROBERT HARINGTON.
From the Department of Pathological Chemistry, University College Hospital Medical School, London.
(Received October 31st, 1928.)
SINCE thyroxine has been available in the pure condition its therapeutic use has been limited by the fact that it is only possible to elicit with certainty the full physiological activity of the substance when it is administered intravenously. When thyroxine or its sodium salt is administered by the mouth absorption appears to be irregular, and scarcely ever is the full effect of a given dose obtained. Except, therefore, in hospital practice where the intravenous injection offers no difficulties, thyroxine compares unfavourably with crude desiccated thyroid gland as a therapeutic agent. In our opinion thyroxine fails to be absorbed after oral administration owing to its extreme insolubility when in the free condition.
europepmc.org/article/MED/1...
Of course, this comes from almost a century ago. Many things have changed. But the insolubility of thyroxine, right up to the development of levothyroxine sodium oral solutions and gel caps (as in Tirosint), has continued to be an issue.
I do not know if Harington revised this opinion in later years.