The underactive thyroid – more complex than just taking L thyroxine.

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People diagnosed with an underactive thyroid are prescribed L-thyroxine in man-made form, examples being Eltroxin/Levothryroxine and Synthroid.  These simple, cheap pharmaceuticals supplies one of the thyroid hormones not being produced plentifully enough by the thyroid.  Yet many prescribed one of these pills still suffer from the symptoms of an underactive thyroid – lethargy, weight gain, feeling cold, rough hair and so on.  The thyroid is a complex organ and this blog looks at four basic thyroid mechanics: what it does, what makes it work, what the thyroid hormone consists of and what the body makes it from.

Firstly, what does the thyroid do?  Two things: it controls the rate of fuel burning in our body and regulates the release of calcium from our bones.  This blog is concerned with the rate of fuel burning; bones and calcium I’ll leave to another time.  Now, the thyroid gland controls our metabolism in the same way as the central heating thermostat controls the temperature in the house.   When the weather is cold, the central heating pumps away, burning fuel to heat the house.  When the weather is hot, the heating does not need to come on and we have smaller fuel bills.  And so it is in our body.  When it is cold, the thyroid is cued to ramp up body temperature, and when it is hot, the thyroid’s activity is turned right down, if not off altogether.  Unlike the house heating which has 1 boiler, our body has billions of tiny boilers in the form of our cells. And to produce energy, our cells use 2 fuels:  glucose and oxygen.  Glucose comes from our food – and insulin is the hormone responsible for telling the cells to how much to burn.  Oxygen comes from the air we breathe and the thyroid hormones are responsible for telling the cells how much oxygen to burn according to needs in the moment.  Just like the boiler in the house, we need our cells burning their fuel well.  Burn too much and we can’t keep body fat on and suffer hyperactivity and anxiety, burn too little and we get fat and tired. However, the ramifications of both problems are much greater in the body than they are in a poorly working central heating boiler.  Of course, if the central heating does go wrong, it isn’t necessarily the boiler at fault, it could be a faulty thermostat.

And the body’s thyroidal equivalent of the thermostat, the chemical that calls the thyroid into action, is called the Thyroid Stimulating Hormone or TSH, released by the pituitary gland.  Too much TSH circulating in the blood is a strong sign that the thyroid is responding only to a thoroughly good prodding and is therefore underactive.

Were there just one thyroid hormone, things would be much more simply controlled by drug therapy.  If we have an underactived thyroid, just give the missing hormone in the right dose and all the symptoms go away.  In fact, the thyroid hormone consists of at least 4 variants: T4, T3, rT3 and T2.  Each coming from the other. To explain the numbers; the body makes the thyroid hormone from dietary intake of iodine and tyrosine. T4 has 4 iodine molecules, T3 has 3 iodine molecules, converting from T4 by the loss of 1 iodine molecule, and T2 has 2 iodine molecules, converting from T3.  How the body converts T4 – 3 – 2 is extremely complex, so, with great relief, we won’t go there.  The reason those with underactive thyroids are prescribed thyroxine is that this is the chemical name for T4 and T4 is the commonest thyroid hormone in the body.  I suppose the doctors hope we just have to hope we can successfully convert this to the other forms.  And where that conversion happens can be another small clue as to why the thyroid could be working less than optimally.

T4 is also the storage form of the hormone which the body converts to T3 in the liver and other tissues.  So problems in the liver may disrupt this conversion process. T4  is slow acting in the cells, taking effect over days. In contrast, T3 is fast acting in the cells and speeds up metabolism or fuel burning;  rT3 is the opposite number and slows metabolism down.  It is formed when T4 converts to T3.   We need both T4 and T3 in the cells.  T2 has been known about for a while, and was thought to be a by- product.  But with deepening of knowledge about our DNA in the cells, T2’s importance is being discovered.  T2 docks into the same receptors in the cells that T3 and T4 dock into and it has been found that taking T2 in drug form helps weightloss in rats. 1This leads to the suspicion that T2 could be the most important thyroid hormone and much research goes on seeing if prescribing T2 will help weightloss in humans.

There is one more complication that is another clue as to why just prescribing thyroxine doesn’t work for everybody with an underactive thyroid.  And that is that T4 and T3 exist in bound and free forms.  In bound form, they are attached to an amino acid, and this is how they trundle around in the blood stream.  But to enter the cell for use, they must be unbound, and so are called free form.

And so there we have it.  The thyroid is responsible for the rate of fuel burning in the body (and controlling calcium loss from the bones), it responds to thyroid stimulating hormone.  The thyroid hormone is made out of iodine and tyrosine and breaks down in the liver from T4-T3-T2 by losing an iodine molecule.  It is used by the cells in free form.  This is a brief overview of basic thyroid mechanics.  I know of many women who have been diagnosed with an underactive thyroid, prescribed thyroid medication, yet still suffer low energy and struggle with their weight.  From this description alone, it can be seen that many things can go wrong with the thyroid and if the medication is not working well, then go back to the doctor and ask for a full thyroid panel.  The excellent book Thyroid Balance by Glenn S Rothfeld goes into details and gives much good advice.

  1. Moreno M, Silvestri E et al.  3.5-Diiodo-L-thyronine prevents high-fat-diet-induced insulin resistance in rat skeletal muscle through metabolic and structural adaptations.   FASB J. 2011 Oct; 25(10):3312-24.  Abstr. []

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