Can hormones such as testosterone, aldosterone and estrogen cross the blood brain barrier? I looked on Wikipedia and there no mention of it in the testosterone article. Through Googling around I also didn't find a list of which hormones can cross the barrier.
Yes, both steroid and peptidic hormones can enter the brain, the obvious proof being that the brain responds to changing blood concentration of hormones.
The situation is different for steroid hormones (such as estrogen, testosterone, cortisol etc) which are small and lipophilic, and peptidic hormones (such as insulin, ghrelin, prolactin etc) which are big and hydrophilic.
The blood-brain barrier (BBB) is selectively permeable to steroid hormones: some of them (such as the gonadal steroids) can readily pass the BBB, while others (such as corticosteroids) pass only in small concentrations. This may however be modulated by hormone carrier proteins in the plasma.
See: Transport of Steroid Hormones through the Rat Blood-Brain Barrier - Pardridge et al., JCI, 1979
When speaking of peptidic hormones, the classic hypothesis is that hormones pass through the areas where the BBB is "leaky", i.e. where capillaries are fenestrated. These include circumventricular organs such as the median eminence (where the terminals of many neuroendocrine neurons are located), or the organum vasculosum of the lamina terminalis (OVLT), important for reproduction and osmoregulation. The chorioid plexus is also often mentioned as a site for hormone entrance and/or sensing.
However, how exactly hormones enter the brain is still very unclear. Our lab published a paper on this topic recently: Rapid sensing of circulating ghrelin by hypothalamic appetite-modifying neurons - Schaeffer et al., PNAS, 2013
Yeah indeed they can. The obvious proof is puberty and development, but a more involved look into how that is achieved can be found in this free article from last year. In particular,
Nearly every mechanism by which a substance can cross or interact with the BBB is used by one hormone or the other. In general, steroid hormones cross the BBB by transmembrane diffusion whereas thyroid hormones, peptide hormones, and regulatory proteins cross by saturable systems.
In particular, I like figure 1: