The Hypothyroidism Numbers Trap: Thyroid Production of T4 and T3 and Conversion Rate of T4 to T3 is Not Fixed

This article is based on an excellent new blog post by the Canadian Thyroid Patient’s group Dr. Tania S. Smith. I will provide the link to Dr. Smith’s post at the end.

I have heard so many pieces of nonsense over the years about how much T4 and T3 is produced by the thyroid gland and what the ratio of this T4 to T3 is. This is often mistakenly assumed to be 80% of T4 and 20% of T3 from the thyroid gland

I have also heard too many times that conversion from T4 to T3 in the peripheral tissues (non-thyroid) is in a fixed ratio also.

Both the fixed thyroid production ratio of T4 to T3 and the fixed T4 to T3 conversion ratio are utter rubbish. They always have been. These vary hugely between each individual. They also adjust within an individual at times.

The problem is that these errors lead to incredibly bad treatment practices, that frequently lead to thyroid patients having too little biologically active T3, i.e. they remain symptomatic at best, and sometimes very ill.

These incorrect beliefs assume that we are all robots that fall off a production line. These beliefs assume we all have identical thyroid gland production levels of T4 and T3 that never alter with circumstance or disease. These flawed beliefs assume that we all have the same conversion ratio from T4 to T3 in the peripheral tissues and that this conversion ratio never alters. The beliefs are utterly flawed and always have been.

Let me provide some examples of what happens in practice. It is not just the endocrinologists and doctors that are in error on this. It is also patient groups and their members as they try to assist other patients using this errant information:

  • When someone is undergoing thyroid treatment their endocrinologist or doctor often assumes that they will always get ideal conversion from T4 to T3 and that simply replacing missing thyroid hormones with T4 will work. This is clearly not always true.
  • When someone is being treated, their doctor may think that they should be adequately treated with a certain amount of T4 medication – because the doctor believes in some of the flawed beliefs on how much T4 is being produced by all people every day.
  • If a thyroid patient wants to add one type of thyroid medication to their treatment, e.g. adding 10 mcg of T3 to T4 medication, they may be told to reduce the T4 level by 40 mcg or so – because of one of the beliefs on how much T4 is equivalent to how much T3. This information might come from a doctor or endocrinologist or another patient on a patient forum. If I had £1 for every time I have heard from a patient that they’ve been told X amount of T4 is equivalent to Y amount of T3, or how much NDT to replace T4 with I would be a rich man.
  • I could list many other examples.

Dr Smith says in her article, “Sadly, such collapse of a wide range of human variation into a single, memorable average has caused great injury to those who don’t fit the “normal man” parameters.

Basically, the variation between individuals of thyroidal secretion of T3 is LARGE.

The variation between individuals of peripheral conversion capability is also LARGE.

On top of all of this, every healthy individual has the ability to make more or less thyroid hormone and convert more or less T4 to T3 in a dynamic way. In a healthy person, with a working thyroid gland, the thyroid hormone system is variable and regulated for them as an individual.

The variation between individuals of other biochemical issues that affect thyroid hormone medication absorption, conversion, and utilisation in the cells (where it counts most) is also LARGE.

This is all frequently made worse when an endocrinologist or doctor insists that TSH can be used to monitor and determine when the ‘treatment’ is adequate.

Every individual needs a unique approach to treatment that is tailored to them and driven first and foremost by symptoms and signs.

Here are some implications for thyroid treatment from the second page of Dr. Smith’s article:

  • People without a healthy thyroid gland are missing half the equipment (the thyroid gland itself) needed to balance the right secretion and balance of thyroid hormones and the right balance of conversion.
  • Thyroid-disabled people do not have the equipment to manage metabolic flexibility. 
  • If our body needs to reduce the T3 supply, that is pretty easy. It is easier to destroy than create. All our body needs to do is trust in Deiodinase Type 3 (DIO3 / D3) expression, which awakens as FT4 and FT3 rise above one’s individual metabolic range. This D3 enzyme will increase the rate of conversion of T4 to Reverse T3 and of T3 into T2.
  • On the other hand, people without a fully working thyroid gland cannot enhance both secretion and conversion at the same time when our bodies need to increase our metabolic rate in response to environmental or health challenges. 
  • For example, we cannot just get more T3 hormone supply in the midwinter cold season when our doctors have given us our yearly dose adjustment in the summer. As a result, in the winter, the Free T3 of patients without thyroids falls significantly lower, while the healthy people get a boost to their FT3 because they can turn up the thermostat to stay warm. Therefore, some thyroid patients may need a backup supply of T3 thyroid hormone. Some patients may need to be on the upper edge of their individual range of optimal thyroid hormone supply so that they can absorb the shock of minor to moderate bumps in the road. 

Dr. Smith’s article explains how endocrinologists and doctors have mistakenly used some older research and have come to invalid conclusions. Individuals have such a massive variability of thyroid hormone production and conversion that simplistic levels and ratios result in extremely poor treatment. Unfortunately, many thyroid groups and individuals offering advice on these have also fallen into the same trap

Here is the actual blog post with the details. It is quite long and detailed but worth the read when you have time to sit down and go through it carefully:

Best wishes,


Paul Robinson

Paul Robinson is a British author and thyroid patient advocate. The focus of his books and work is on helping patients recover from hypothyroidism. Paul has accumulated a wealth of knowledge on thyroid and adrenal dysfunction and their treatment. His three books cover all of this.

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