There are two aspects to this issue: transporting/storing hrts and using them, and both are affected by heat and humidity.
HRTs and heat
All hrts are affected by temperature and humidity to some degree.
That is the notional reason why hrts have expiration dates (although that period may also be affected by marketing and sales). Further, pharmaceutical manufacturers only guarantee their products' effectiveness within the range of 68-77F (20-25C) although pharmacists tend to support the broader range of 58-86F (14.4-30C) as non-damaging.
Estrogen itself may lose potency when heated, although it's difficult to find specific references that detail this process and much of it is cloaked in spurious dietary advice. Nonetheless, it's a complex molecule and it only works when undamaged.
Irrespective of whatever happens to the estrogen molecule itself with heat, though, other aspects of specific hrt deliveries can definitely be adversely affected by heat.
Oral tablets, whether meant to be swallowed or used transbuccally, will lose more potency over time at high temperatures than cool ones. High humidity getting into pill bottles may also cause tablets to crumble or capsules to stick together and break when attempts are made to separate them. This can affect dose strength and thus how our needs are covered by our usual dose.
Creams may melt and separate in the heat. Sure, you can stir them back together, but that's still iffy in terms of the exact distribution of the active ingredient through the vehicle. Gels may be less well--absorbed if their alcohol-based vehicle evaporates so quickly in the heat that we can't spread them to the required area size, and they may become more concentrated as alcohol evaporates out of the container. Sprays and lotions are liable to suffer similar effects that can change potency of a dose, whether by affecting dilution or application.
Over the years, we've heard of a number issues to do with patches and heat. A common one is the effect heat can have on the adhesive, which is necessary for the delivery of the estrogen the patch contains. Patch brands that previous adhered fine may no longer stick for the full dose period or may no longer adhere fully.
Where HRTs run into heat (and cold) problems
Let's segue from patch adhesion concerns due to adhesive damage into the other issue to do with using hrts in the heat: our bodies. There are two things going on here.
First, there's sweat. Even an otherwise fully adhesive patch may falter when placed on sweaty skin. Similarly, sweat may dilute other transdermals like creams, gels, or sprays, changing the spread area and uptake speed.
Beyond that, though, heavy sweat after application can cause a patch to lift. It can also wash away (or transfer to others) estrogens cached on top of the skin, such as those applied by gel or spray. Only oil-based creams penetrate the skin fully at time of application and are not affected by later sweating. Now, this obviously may not apply to a light "glow" but when we spend an hour or a day or days and nights pouring sweat, then we're dealing with a substantial possibility.
HRTs in transit and storage
But not all damage to hrts comes while we're applying them. They also have to get into our hands in the first place and then from there into/onto our bodies.
HRTs are shipped from the plant where they are manufactured, stored in a warehouse, redistributed/retransported once or several times by a pharmaceutical chain, and stored in the pharmacy all before they come to us, after which we may carry them around or leave them in our car parked while we carry out other errands. Additionally, if we use a mail-order pharmacy plan, the prescription mailer is stored and handled by a shipping service, spends a day or more being driven around in an open delivery truck, and then spends time in our mailbox, which itself may be outdoors or in a temperature-uncontrolled space. Can you see all of the times and places where your prescription could bake in the heat (or freeze in the cold for winter refills)? Because most hrts don't require refrigeration, they may not be handled in ways that preserve that relatively narrow range of effective temperature. And there's no way for us as customers to know that these things may have happened.
And of course, once we do get them home, many of us are finding that where historically we didn't need air conditioning to get through summer weather, we're now living in much hotter temperatures. Our homes may spend days or even weeks at the elevated levels that damage hrts.
So what can we do? We have no control over what happens before we're holding our hrts in our hands, and the many commercial interests that control them upstream of us have, at the moment, little incentive or regulatory interest in minimizing damage. Similarly, we've not heard much about insurance refusing to cover replacements of heat-damaged hrts. But as this problem grows, we might expect them to move to protect their profits by making new policies to limit refill coverage. This comes down to fiscal politics, which is rather outside the purview of this website but certainly a concern for many of us in the world we live in.
But once we've got control over our hrt, we can take steps to preserve what potency it might have.
If you're using a mail-order service, is that negotiable with your insurance? Can you document heat damage (photos are your friend, here) to packaging or mailboxes in the hot sun? Can you arrange quicker pickup of the mail or move the mailbox to a shaded location? Sometimes once we figure out where problems can occur, we can take steps to reduce their likelihood.
If you're picking your prescription up from the pharmacy, you may be able to avoid leaving it in a hot locked car while you finish your shopping. Basically, if you wouldn't leave your child or your pet in that locked car, it's also too hot to leave your hrt.
What about at home? To begin with, the bathroom is the worst place in the house to keep your hrt because of all rooms, its the most likely to become even hotter and even more humid as you use it. So yeah, it may be convenient...right up until your hrt doesn't function well enough any more.
Going in the other direction, refrigerators are cooler than the recommended temperatures, so they aren't the best choice either. Still, if we were trying to decide between a 100F+ room and a refrigerator, we'd seal our hrt up well (to prevent moisture damage) and put it in the fridge. If you have a basement, that may be slightly cooler if you have access to a secure location in it. You may need to search to find the coolest place in your home and even then it might not be cool enough. But it's still good to get those hrts out of the bathroom.
Obviously, if you have air conditioning that's going to help a lot. But many of us don't have the money or even the time to put this in place, especially if we don't own where we live. In a pinch, there are a number of clever cooler concepts that have been developed for third world countries that can be adopted to create small, custom coolers or to cool the air without electrical appliances (1, 2, 3).
HRTs in use in the heat
It seems obvious that we should do what we can to keep our use of hrts as unaffected by the heat as possible. But when our bathroom is roughly the temperature of the surface of the sun, it may be difficult to actually dry our skin off enough to apply an hrt. Additionally, because our pores are open and blood vessels close to the skin are dilated to the max to try to dump excess body heat, our bodies may take up our hrts much faster than in cooler weather. Both of these things may affect how we feel on our hrts.
What to do? Once we're aware of the issue, we need to do some personal troubleshooting. Is there a (slightly) cooler part of the house we could go to and cool down a bit before applying our hrt? Can we stand in front of a fan until our temperature normalizes a bit? Should we even go so far as to switch the time of day of taking our hrt, even knowing that this might put us in conflict with our natural biorhythm of higher estrogen levels in the morning? Do we need to take special precautions around kids or pets to avoid sweating some of our hrt onto them? All of these will require personal answers and troubleshooting, but they're things to consider. The inconvenience of changing our routines for hot weather (and then remembering to actually do them!) may well be offset by not having to endure hormonal instability. Overheating is annoying enough without adding hot flashes and mood instability from fluctuating estrogen.
Beyond keeping things cool, sealing pills or sprays in a plastic bag might help protect them, a bit, from either humidity (pills) or excess evaporation (spray canister).
How will you know if your HRTs are heat-damaged?
Well, some signs are obvious. If you open your mailbox and find a stained parcel or the hrt packaging itself shows signs of leakage, taking a photo and then taking steps to return that hrt is a good place to start.
But mostly, it's not going to be obvious. It's more likely to show up in how our hrt works for us. If we've been stable on this hrt at this dose for a time and suddenly things are falling apart, it's not that our hrt has "stopped working" or some other magical effect has taken place ("jammed receptors" or "needing a hormone holiday" are some of the popular but incorrect mythologies). It may be that our hrt just isn't delivering that dose due to heat damage or heat effects upon delivery. But this is a tricky call, one that takes some thought. Extreme hot weather is a stressor in itself, and we know that stresses, especially ones that linger on for a week, two weeks, or more can also upset our hrt balance. But as we troubleshoot loss of balance in hot weather, we shouldn't forget the heat/hrt damage as one possible cause.
So should we just double up on our hrt in the heat? No, that's likely to be a too-sweeping change. Really, a better first option would be to try to get a replacement refill that hasn't been damaged and to store it as appropriately as we can manage. But if we're stuck with a damaged batch and need to make the best of it we can, it's probably going to be less stressful and more on-target to slowly increase the amount we're using by no more than 10% of a usual dose at a time. That lets us feel our way to "right" rather than risking the even worse discomforts of an excessive dose. And, of course, it's important not to forget to revert to the correct dose again with the next refill. Even if it's still hot? Yes, because we won't know precisely how damaged this one is, and the adjustment for one batch may not at all be the adjustment needed for the next.
Special cases
This also applies to our summer vacations. We may have fine temperature control at home, but how about when we've packed the whole family in the car and headed off to [vacation destination] with our hrt stashed in a suitcase in the trunk? Taking care to keep our hrt within temperature range during travel and at our destination is a annoyance, but also can go a long way towards reducing the risk of losing our hormone balance while we'd rather be having a nice time on vacation.
And one last word on wildfires and other climate disasters. With increasingly extreme heat comes dryness and risk of fires, often in heavily-inhabited areas, or more violent hurricanes affecting areas further inland. The wise thing to do as our climate changes is to have an evacuation plan, if not "go bag" with essentials (this is a basic example of what to include, but an online search will reveal many more articles and lists). While we may not keep our prescriptions in them, having a grab-at-the-last-minute checklist so that we're reminded to take them with us can be essential to our health. Losing a home or being relocated is traumatic enough; in an evacuation scenario, it can be extremely difficult to get needed prescription refills and in some cases, insurance companies may not deem that an adequate justification for them. There are many resources online about how to put together evacuation bags, but for those of us on hrt, that's a critical piece to include.
Plan ahead
No matter where you are in your annual climate cycle, we can look forward to heat, extreme weather, and wildfires becoming the new norm as our planetary atmosphere grows ever warmer and more violent. We all need to be aware of the special vulnerability our HRTs represent and take steps to plan to protect them as much as we can.
