Where to begin - The Finnish study is retrospective and looking at data already recorded in their systems. Those in the treatment group chose to have puberty blockers and those in the control group did not receive puberty blockers. So they can legitimately state that psychiatric illness was more common in those who took puberty blockers. They followed people over time. Therefore they can look at how changes in the mental health of those who took puberty blockers compared with adolescents who did not. Their control group was not a self selecting group.
The puberty blocker trial is not retrospective. Therefore it has to be designed to higher standards. The comparison group would not receive the same treatment as those taking puberty blockers, they would receive extras and that included extra payments for participating. So the difference would not only have been the effect of puberty blockers. It was a trial of alternative treatments in which one was known to cause harm and to cause harm to more people than it would have helped. Most children are known to desist if not given puberty blockers so more children taking puberty blockers would be harmed than would be helped. How many children is it ethical to knowingly harm because a treatment may help others when you have only anecdotal information to say that it does?
The NHS holds a lot of data on young people who have been in contact with health professionals. That data covers not only mental health contacts, as in the Finnish study, but also their contacts with other health care staff. The Tavistock staff were discouraged from keeping good records, other staff were not. There is valuable information there that those who want children to continue being harmed do not want the NHS to explore. Fortunately the Finns didnt listen to those saying retrospective studies are not perfect, they analysed their data anyway.
If anyone actually believed that puberty blockers really help children they would be encouraging all those who took them to allow their data to be included in a retrospective study because then there would be more chance of showing that people had been helped. TRAs discourage participation because they dont want the harms exposed.
Personally I'm willing to believe that some children may have been helped by puberty blockers and even that the mental benefits are capable of outweighing the physical damage - but in a very small number of children. Its not ethical to run a puberty blocker trial without finding out how small and what the characteristics of that group are. Once you've done that you might be able to make an argument for an ethical trial, until then you are knowingly harming children and even paying them to harm themselves. You dont know which ones you will harm but you know it's more than you help. The state is also in loco parentis for some of these children as children in care make up some of those involved.
Ask the Finns to try and identify if there was a subgroup whose mental health did not deteriorate and maybe you have a chance of an ethical trial but you still need to collect data on the physical damage.