Weather can affect our thyroid hormones. If it is too hot, we can need less and when too cold we might need an increase.
I don't think steroids have an affect on NDT as steroids are also 'natural' to our bodies. This is an excerpt:
While the potency tends to be fairly stable within a particular brand name -- or generic manufacturer -- they do vary from brand to brand and manufacturer to manufacturer. Still, if you're stabilized on one brand, shifting to another brand -- or being on generic levothyroxine and having refills come from different manufacturers -- can cause some swings, based on the different potencies of each maker's drugs.
Another hitch? Hot weather can degrade medications in general, and thyroid drugs are particularly susceptible. So, mail-order medications that sit in hot trucks and mailboxes for hours and days, drugs that sit in hot cars or storage in hot homes without air conditioning can all have a fairly quick and dramatic effect on potency of medications. (For more information, read Warning: Hot Temperatures May be Hazardous to Your Drugs: Your Medication Can Be a Casualty of Heat Waves and Power Outages.