As in, if it's hot one day and cold the next, or vice versa, you might find yourself more sluggish, or stressed, or just generally jagged up.
Your thyroid releases hormones that regulate how much energy you burn for heat, and how much for general activity. That's why people with underactive thyroids are often chronically cold, and people with overactive thyroids overheat easily.
The thyroid hormone T4 has a half-life of 5-7 days in the body, and T3 has a half-life of 1 day. Meaning, once it's set your levels, if the temperature suddenly drops, it can't really adjust on the fly. Your liver can re-absorb some of the hormones but not very quickly.
Your body has other ways of regulating temperature - sweating, flushing, shivering, etc. - but the baseline is set by the thyroid. And in terms of general happiness, you're already pretty miserable by the time you're shivering / red-faced & sweating.
I think of it as jetlag - your body can't respond quickly enough to the change, and it's out of sync. And like jetlag, it's not as simple as "I feel sleepy at awake-times" - you feel generally just hit by a brick (also like jetlag, some people are breezily unaffected by it, the bastards).
You would also know that if you do not live in the tropics and then you visit there for a week, you will probably feel exhausted. But if you move there longer term, you adjust.
Anyway, not much you can do about it except be kind to yourself if you feel jagged up for no reason (and then you remember there was a big temperature change).
Please note that this was a bit simplified: the thyroid does lots of other things, the thyroid system involves some other glands I didn't mention, there are many other reasons why you might feel the cold/heat more than other people, and thyroid function still has many unknowns to it.
Here is a study on seasonal variation in thyroid hormone levels. My impression is that if you live in a place with relatively stable seasons - cold for ages then warm for ages - you'll feel better.
Bit about Melbourne weather, niche audience
Melbourne weather is Like This basically because we have this huge hot desert in the north and freezing winds blowing straight from Antarctica in the south, and there are no mountains to slow down the tug of war between them. It's terrible. No discussion will be entered into on this last point.
This piece was originally published in The Whippet #70 – subscribe to get the next one in your inbox!
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