Taking levothyroxine & overheating in hot weather - the penny bounces back up.

Oh, whoops!

Two weeks after discontinuing levothyroxine, I was still feeling too hot (we're still having a heat wave). I thought that maybe my pituitary had started to function and my thyroid had started to produce thyroxine. I checked my body temperature. It was 35.3°C (95.5°F). The penny bounced back up.

How hot I feel ≠ How hot I am.

I'm back on 125μg/day of levothyroxine.

Taking levothyroxine & overheating in hot weather - the penny drops.

Last night, I was sitting in a hot stuffy pub listening to some excellent music being played at a jam session. During the breaks (when it was quiet enough to hold a conversation), I was chatting with Jack the Rapper. As I was chatting, a thought popped into my head about why I was dripping with sweat, when everyone else in the audience wasn't. Here it is.

The Hypothalamus secretes TRH (Thyrotropin Releasing Hormone) which reaches the Pituitary via the hypophyseal duct.

The Hypothalamic Pituitary Thyroid Axis (HPTA) regulates body temperature. It varies thyroid hormone levels (T4 & T3) which varies Uncoupling Protein (UCP) expression, which varies heat production, which varies body temperature. See Minimal changes in environmental temperature result in a significant increase in energy expenditure and changes in the hormonal homeostasis in healthy adults.

"Thyroid hormones axis Compared with exposure to 24 °C, exposure to 19 °C resulted in small, non-significant increases in total triiodothyronine (T₃) and TSH AUCs and a significant increase in serum free thyroxine (T₄; P=0.03). When the analysis was performed according to the gender, a small but significant increase in serum T₃ AUC was observed in males (P < 0.05) but not in females. Similarly, while the change in free T₄ was highly significant in males (P < 0.002), no significant change was observed in females." Is this why women feel the cold more than men?

My Pituitary doesn't secrete Thyrotropin a.k.a. Thyroid Stimulating Hormone (TSH), so I'm prescribed levothyroxine 125μg/day every day. When the ambient temperature rises, my HPTA doesn't lower my thyroid hormone level as I am running "open-loop". My UCP produces too much heat which makes me overheat. Ker-ching!

From now on, I will adjust my levothyroxine dose to a value where I feel comfortably warm at all times. According to http://www.drugs.com/pro/levothyroxine.html, the half-life of levothyroxine is 6-7 days. Any change in levothyroxine dose will take about a week to get half-way to its final effect on internal heat generation. This could take quite some time!

Continued on Taking levothyroxine & overheating in hot weather - the penny bounces back up.