I have a distinct disliking of equalling concepts and deities of one culture/religion with concepts and deities of another.

There may be similarites, yes, of course there are for deep down beneath all the layers of culture, religion, tradition, habits, memory, psyche or what you will, we are all human beings with the same basic needs, hopes whishes, fears, drives.

But saying that one is the same as another, as I hear quite often, is to deprive both cultures/religions of their uniqueness and is a subtle claim that humans have no discernment in forming their conceptual worlds.

Therefore: the link between Thoth and Hermes Trismegistus, is weak to say the least, I boldly state ;) Following is an effort to go to the bottom of this particular issue, without claiming to be complete.

Hermes Trismegistus
The figure of Hermes Trismegistus belongs in an old western wisdom-seeking tradition which is known as Hermeticism and which embodies several different texts. It is related to both Gnosticism and Alchemy. All these texts are aimed at the ennobling of the human spirit.

The figure of Hermes Trismegistus is traced back to the Greek god Hermes, with the word 'Trismegistus' meaning 'Thrice Great', which is a Greek addition done at some point. But when did this happen approximately? This page talks about there being three different sources from which the name of Trismegistus might have come and speculates on the origin of this enigmatic figure. According to some sources, it originated as the title, "Great, great, great", which appeared on the Rosetta Stone (Williams, 691).

Text of the Rosetta Stone where it says Thoth the great and great.

According to other sources it came from a rendering into Greek of an Egyptian epithet used to refer to Thoth - the Egyptian "aa aa ur" became the Greek "megistos kai megistos theos, megas Hermes", meaning "greatest and greatest god, great Hermes", and which evolved over time to the shorter "Hermes Trismegistus", "thrice greatest Hermes" (Mahé, 287).

Hermes
Hermes is considered the Greek variant of Djehuty. Here is a pretty good page about that. This begs the question: how did the Greeks interprete the ancient Egyptian god Djehuty and what were their sources for this interpretation?

Hermes is a deity in his own right, a god of boundaries, a messenger, and inventor, even a thief and trickster. I claim that he is quite different that the Egyptian Djehuty. Hermes Trismegistus is something else again. Maybe he can be seen as an older, wise and mature version of the young Hermes?

Djehuty: (Gr: Thoth)
While not going into details or a full description here, Djehuty was early on represented in two forms: the sacred ibis, (Threskiornis aethiopicus) which appeared on a standard on slate palettes of the late Predynastic period, and as a baboon (Papio Cynocephalus), on glazed figurines at Abydos, from the 1st Dynasty.

He was considered the 'lord of the sacred words', who gave to the Egyptians the ability to read and picture symbols, i.e. write. Therefore he was also the embodiment of all scientific and literary achievements and the 'House of Life', i.e. the Per Ankh, was a vast resource of knowledge and records.

He was a judge and acted as an upholder of Ma'at, he protects Isis, he arrives at her bidding to heal the Horus child when bitten by a snake in Chemmis, he gives back Isis her head when she loses it in the struggle between Horus and Set, where he also functions as the secretary of the Ennead as well as heals the Eye of Horus when it gets wounded in the fight.

In other texts he is claimed to have been created 'from the lips of Re' to uphold Ma'at. He used his baboon-form to shine during the night, and in his ibis-form he acted as the messenger between heaven and earth. In one myth he made the Eye of Re in her form as Sekhmet, to return to her father Re.

With this as background, I have some difficulty in seeing just how Hermes Trismegistus can be traced back to the Egyptian Djehuty (Thoth), other than as a general effort at Man´s search for Truth and Wisdom. That is well and good as it stands. But why did the Greeks interprete Thoth as their Hermes at all? What were the grounds for this? One reason is the fact that Thoth in myth is regarded as a go-between and messenger in the Contendings of Heru and Set. I only partly agree with this as it puts a too narrow definition on Thoth. Thoth is not a messenger per sé, he acts as a mediator and on his own initiative. He is not sent out by the sun-god on an errand, rather, in his capacity as a moon-deity he acts as the legitimate deptuy of Re. That is quite a different matter and it reduces above Greek reason to a rather narrow interpretation.

Greeks and Romans like Herodotos and Plutharch, viewed the ancient Egyptian culture through their own culturally coloured viewpoints. They judged and understood the ancient gods and world view on different terms than the Egyptians themselves did. Much was lost on the symbolic connotations and underlying principles. Much later the Gnostics and Alchemists came along and did their own interpretation, coloured by European history and development during over a thousand years after Egypt, and indeed the Roman Empire too, had disintegrated. I´m very suspicious that there was ever anything left of true Egyptian origin in the Alchemistic works. Rather they lean on a Greek interpretation of the world and of old Arabic source material.

One of these Hermetic texts is called the Emerald Tablet and is considered to be probably the earliest Hermetic texts found sofar.

Excerpts from: History of the Emerald Tablet:

"The Tablet probably first appeared in the West in editions of the psuedo-Aristotlean Secretum Secretorum which was actually a translation of the Kitab Sirr al-Asrar, a book of advice to kings which was translated into latin by Johannes Hispalensis c. 1140 and by Philip of Tripoli c.1243. Other translations of the Tablet may have been made during the same period by Plato of Tivoli and Hugh of Santalla, perhaps from different sources.

... the Kitab Balaniyus al-Hakim fi'l-`Ilal (book of Balinas the wise on the Causes). It has been proposed that this book may have been written as early as 650, and was definitely finished by the Caliphate of al-Ma'mun (813-33)."

By c AD: 450 noone could read or write hieroglyphs any longer. This is when the Bysantian emperor Flavius Anicius Justinianus closed both the old school of philosophers in Athens and also the last functioning Egyptian temple, that of Isis on the island of Philae in Upper Egypt. This is where the priesthood still knew how to read and understand the hieroglyphs, although the rest of Egypt had turned to Christianity.

At the time of 800 AD, Egypt had long since been a Muslim country and the only travellers who succeeded to get into Egypt were Crusaders. They seldom got beyond Cairo. Where would they have learned the ancient language?

Maybe it is that when people have looked at the hieroglyphs they have always felt some kind of intrigue and mystery, they have wanted to explore them and understand them. That wasn´t possible until the early 1800´s and the work of Champollion. The tradition of alchemy doesn´t go back further than to 1200-something, info about that and Hermeticism abounds.

So there´s a big gap of over 1000 years, from 450 to 1822 when Champollion announced that he had made a breakthrough in dechifering the hieroglyphs on the Rosetta Stone, where noone knew how to understand the hieroglyphs. Which lays the road open to all kinds of misunderstandings and assumptions.

After this it was possible to read that Thoth was called 'great-great' on the Rosetta Stone and that Ptolemaic text talked of him as 'five times great'.