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Today’s post was requested (quite a while ago) by both Siobhan and Matt, and it’s about how hypothyroidism works. Hypothyroidism is a condition that arises when the thyroid gland just cannot get motivated, and lies around all day consistently not producing enough thyroid hormones to do all those tricky jobs that thyroid hormones need to do. In order to understand how hypothyroidism works, it’s pretty key to find out what thyroid hormones do. And actually, understanding that also explains how hyperthyroidism works too.
The life cycle of a thyroid hormone
The thyroid hormones, triiodothyronine (T3) and thyroxine (T4), are produced by follicular cells in the thyroid gland. They only do this when under strict instructions from another hormone though, the boss of thyroid hormone production, thyroid-stimulating hormone (TSH). TSH carefully regulates the amount of thyroid hormones that the body produces; when there’s a lot of T3 and T4 about, it instructs the thyroid to slow things down, and when there’s very little T3 and T4 to be seen, anywhere, it takes up a megaphone and startles the thyroid into rectifying that immediately. Unlike most hormones, T4 and T3 don’t peak at certain times; in fact the very raison d’etre of TSH is to keep the level of thyroid hormones as steady and constant as possible.
When TSH does need to produce hormones, it stimulates follicular cells, and they absorb a protein called thyroglobulin. This protein has been captured for use by an enzyme, thyroid peroxidase, which traps the protein with iodine. It then passes it to the follicular cells, who strip the protein for parts; taking the iodine-trapped residues of tyrosine amino acids from it, and using them to form T4 that can then be sent out into the big wide world, or the if wide human body, at least.
As hormones go, T4 has a lovely life, really. Once produced, it whizzes around the body via the circulatory system by hitching a ride with passing proteins. Usually, a protein called thyroxine-binding globulin (TBG) takes pity on it, idly hanging about by the roadside, or blood-vessel side, and picks it up. Once it’s piggy-backing on TBG, it is are inactive, so it can ride around the place for ages, just sight-seeing, not getting up to much. However, when necessary it can let go of TBG and exist in the bloodstream as free and active T4. This is when it is needed to make changes, be proactive and generally Get Important Things Done.
However, T3 is actually quite a lot more efficient than T4 at Getting Important Things Done. T4 is a bit of a hitch-hiking loafer, but T3 is highly motivated and ready for action; so when T4 reaches the place it needs to be, it is converted to the hyper-efficient T3 and that’s when the Important Things starts Getting Done.
Thyroid hormones bind to a thyroid receptor, and in doing so trigger a series of cellular signals that lead to changes. They are the hormones responsible for regulating metabolism and energy balance; amongst other things they increase the metabolism of carbohydrates in the body, increase heart rate and cardiac output, increase the body’s base metabolic rate, and increase the effect of catecholamines in the brain. They basically speed everything up a bit, and keep things running. The fact that they are so important is why it can be such a big issue to have not enough of them… or too much.
Hypothyroidism occurs when the body cannot produce enough T3 and T4, there are a variety of possible reasons for this, one of which is a lack of iodine. Considering how crucial iodine is to making thyroid hormones, you can imagine that a serious lack does inhibit production, with some seriously rubbish effects.
When the body cannot produce, for whatever reason, enough thyroid hormone, metabolism slows down. This typically results in weight gain, a feeling of tiredness, slow heart-rate and a real intolerance for cold temperatures. A lack of thyroid hormones being produced can also cause the thyroid gland to swell to form a lump or goitre, in an attempt to solve the problem; however if the problem is that there is simply no iodine to be found, no amount of thyroid growth will help.
By comparison, some thyroid glands are just too excited. Instead of not producing enough thyroid hormone, they produce far too much. The symptoms are very much the opposite as well, instead of slowing down, your body reacts as if someone had hit a fast-forward button; you may get heart palpitations, excessive sweating, weight loss, and muscle weakness. Your body is just trying too hard.
In both cases, the carefully regulated levels of hormones are all over the shop. This is the real wonder of hormones, the tiniest fluctuation matters; everything is balanced perfectly, on the edge of a knife. One wobble, and everything can go wrong.
*PLEASE NOTE, AS EVER, I AM A SUPER-ENTHUSIASTIC NERDY GIRL WHO LOVES FINDING OUT THE ANSWERS TO QUESTIONS, I AM NOT A DOCTOR IN THE SLIGHTEST. IF YOU HAVE ANY CONCERNS ABOUT YOUR THYROID, OR YOUR HEALTH IN GENERAL, YOU SHOULD BE SEEING YOUR GP, NOT HANGING AROUND LISTENING TO ME WANG ON ABOUT CAKE*