Three genuinely different kinds of molecule sit under this heading, and the difference is worth getting straight because it changes what any given study is testing.
The chain
The hypothalamus releases growth hormone releasing hormone. That prompts the pituitary to release growth hormone. Growth hormone then acts throughout the body.
Three links, and a compound can be aimed at any of them.
Secretagogues
Ipamorelin sits here. A secretagogue prompts the pituitary to release the body’s own supply, which means the output stays under whatever regulation the body normally applies. Selectivity is the property researchers care about, because this part of the endocrine system has a lot of crosstalk in it.
Releasing hormone analogues
Tesamorelin sits here, one step further upstream, mimicking the hypothalamic signal that starts the sequence.
Growth hormone itself
Somatropin is the hormone, not a prompt to release it. A completely different kind of intervention and a completely different regulatory position.
Why the distinction matters
A study using a secretagogue and a study using growth hormone are answering different questions, and results from one do not transfer cleanly to the other. Reading which was actually used is the single most useful habit in this literature.
Regulatory position in the UK
Several compounds in this area are licensed prescription-only medicines. We state that clearly on the relevant product pages. All are supplied as laboratory reagents for in vitro research.