Across the gland boundary
Thymulin and pituitary signalling
The thymus sends a signal into a conversation that can influence the thymus in return.
ACTH release in culture
Hadley and colleagues studied zinc-bound thymulin in rat anterior pituitary cells. They reported stimulated release of immunoreactive ACTH, together with changes in cyclic nucleotide formation. The preparation matters to the question: the observation came from pituitary cells studied in vitro.
To me, that response makes the thymus an endocrine participant worth taking seriously. The effect was measured in a preparation from another gland, which makes a useful challenge to the habit of assigning an organ a single job. Yet the result is still local to that experimental setting. An ACTH response in cultured rat cells cannot settle how a whole person would respond.
A narrower question now seems much more interesting than whether thymulin is good for hormones: under which conditions does this interaction occur, and what regulates it?
Production under endocrine influence
The 2009 review describes thymulin production by thymic epithelial cells as strongly influenced by the neuroendocrine system. In the same account, thymulin acts as a hypophysiotropic peptide: one that acts on the pituitary.
My first picture was a message leaving the thymus and arriving elsewhere. This account asks for a more restless picture, because the production of the message is itself subject to endocrine influence. The return direction changes the question. Measuring a response at the receiving end tells only part of the story; the sending end also has a regulatory life.
That is why reciprocal influence seems a better subject for this site than a catalogue of hormone effects. The conditions around a signal now seem as interesting as its destination.
The developmental experiment
A study of neonatal thymulin gene therapy in congenitally athymic female mice reported prevention of deficits in circulating LH and FSH and of ovarian dysgenesis. These mice were born without a thymus; ovarian dysgenesis means abnormal ovarian development. The intervention used an adenoviral vector expressing a synthetic thymulin analogue.
The reproductive-development result makes the endocrine connection larger and stranger. Its unusual starting condition matters just as much. I see a way of probing the consequences of a missing thymic signal in these animals, with much still to establish about ordinary endocrine regulation.
The replacement-experiment essay stays with that starting condition and the intervention used to change it.