The thyroid is a gland that sits at the front of the neck. It has two halves, with each half sitting on either side of the windpipe at the front of the neck. Being a gland, it is part of the endocrine system – a series of highly complex glands and structures that produce and secrete substances (often hormones) into the blood, or other parts of the body.


The thyroid is one of the largest glands in the body, and the name ‘thyroid’ comes from the Greek word for ‘shield’ (the shape of the cartilage above the thyroid)

Underneath the thyroid gland there is a series of small parathyroid glands, and whilst these play a vital role in our physiology (mostly through calcium regulation), the focus of this article will be on the thyroid gland.

What does it do?

The thyroid is the main control centre of our metabolism! It plays a central role in regulating our overall metabolic rate, through controlling temperature, energy production and expenditure, and also controls how sensitive our body is to other hormones and how efficiently various metabolic processes occur. Later in the article there is a table of signs/symptoms of thyroid disease, and you can see just how many effects this one gland has!

How does this process work?

It all starts in the brain, specifically, an area called the hypothalamus. The hypothalamus releases a substance called thyrotrophin releasing hormone (TRH). TRH then travels to another area of the brain, called the pituitary gland, where it gives the instruction to cells in the pituitary to make a hormone called thyroid stimulating hormone (TSH). This TSH is then released into the bloodstream, where it travels to the thyroid gland.

Once at the thyroid, TSH causes the thyroid to start producing its own hormones, triiodothyronine (T3), and thyroxine (T4). The actual synthesis of T3 and T4 is a very complex physiological process, however it is worth knowing that iodine is crucial to the process. T3 and T4 are made largely from iodine, and the numbers ‘3’ and ‘4’ indicate how many iodine atoms are used in each thyroid hormone molecule.

Once they have been synthesised, T3 and T4 are then released into the bloodstream, where they can travel to the sites at which they will exert their effect. Although T4 is produced in much larger amounts than T3, it is T3 that exerts the strongest and most significant effect on cells. To help combat this imbalance of production, T4 is largely converted to T3 once it has left the thyroid. This is done by an enzyme that removes one of the iodine atoms from T4, thereby turning it into T3. Interestingly, TSH (the hormone that gives the thyroid the instruction to produce T3/T4) can also assist in performing the aforementioned conversion!

Once it has been converted to its active form, and arrives at its destination, T3 then crosses the cell membrane (the lining of the cell) and binds to a receptor which is located deep within the cell. Once this binding has occurred, it initiates a sequence of events which determines how the cell functions, and how quickly it will do so.

Some proteins act as carriers – meaning they transport substances around the body. Thyroid hormones are a perfect example of a ‘passenger’, in that more than 99% of circulating thyroid hormone is ‘bound’ to proteins, and it is the ‘unbound’ portion of the hormone that exerts its effect.

It’s all about feedback!

In a healthy thyroid, the production of T3 and T4 is kept under relatively tight control – if this process becomes disrupted, thyroid disease can manifest. You will recall from earlier in the article that the pituitary gland produces TSH, which then travels to the thyroid, giving the instruction to synthesise T3/T4. Well, the pituitary gland is also capable of detecting the circulating (blood levels) of T4, and will adjust the amount of TSH it secretes depending on whether levels are high, low, or normal. This is an example of a ‘negative feedback loop’ and is much like the thermostat in a heater.

When the temperature in the room reaches the set temperature, the heater will switch off; when the temperature falls below the set-point, it will turn back on. The same principle applies here – if T4 is low, more TSH is produced, and conversely, if T4 is high , less TSH is produced.

Signs and symptoms of thyroid dysfunction

Weight gain, despite no obvious caloric excess
Weight loss, often unintentional, with increased appetite
 Feel cold easily  Feel hot easily
 Heavy periods (women)  Irregular, light periods (women)
 Tired, lethargic  Tired, but restless
 Depression & slowing of thoughts  Anxiety, irritability
 Constipation  Diarrhoea/loose bowels
 Dry, coarse hair  Thinning of hair
 Slow heart rate  Rapid heart rate


Hypothyroidism, or ‘underactive thyroid’ is something you may encounter with your clients, as it often leads to weight gain, lethargy, and a host of other metabolic signs and symptoms (see table above). Hypothyroidism also leads to (potentially) a host of very serious metabolic consequences, including derangement of insulin function/carbohydrate metabolism, elevated blood lipids (cholesterol), and elevated levels of ‘stress hormones’ such as cortisol. Many of these metabolic derangements precipitate vicious cycles that further impair thyroid function, and therefore worsen metabolic damage – the cycle is fuelled by its own actions!

There are many causes of an underactive thyroid; medications, diseases, radiation, diet induced, hormone induced, ‘lifestyle induced’ and many more. A common cause that you may encounter amongst your clients and potential clients, is diet induced hypothyroidism.

Thyroid ‘burnout’ and diet induced hypothyroidism

Often in an attempt to lose fat, people will restrict their calories – often dropping carbohydrates to extremely low levels. The problem with this is that over time, this will induce ‘metabolic slowdown’. This occurs because their body is not receiving adequate calories to fuel the numerous metabolic processes that are required to alter body composition, and maintain health.

As a result of this, metabolic processes slow down drastically, the body tries to accumulate fat - it is preparing for a prolonged period of starvation; a survival mechanism. The poor old thyroid tries to ‘speed things up’ for the whole system, but the end result is sluggish and diminished thyroid function. This can be likened to us sprinting – it can only be sustained for a very short time before we become exhausted, and even incapable of functioning/performing at a ‘normal’ level, until we have adequately recovered.

The restriction of calories is bad enough, but is also often combined with excessive amounts of exercise. These two, in combination lead to high levels of cortisol. Poor sleep, hectic lifestyles, and some medications also cause elevated levels. Cortisol is a hormone that is great at causing muscle loss, weight gain, depression, impaired immunity, and deranged insulin metabolism (as well as many other effects). Not what we want. Cortisol also effects the conversion of T4 to T3 – remember T3 is the active stuff. If all this wasn’t bad enough, cortisol will also cause the pituitary to release less TSH, further worsening the underactivity of the thyroid. The carbohydrate restriction that usually accompanies these diets leads to a reduction in insulin; this again worsens the situation, as insulin helps convert T4 to T3.


This is one of the most common forms of thyroid dysfunction, and is the medical term for ‘overactive thyroid’. It occurs when the thyroid releases too much T3 and T4 into the bloodstream, causing the metabolic rate to become too fast and chaotic. Generally we think of acceleration in metabolic rate being a good thing – however in this instance it is uncontrolled and dysfunctional, leading to more harm than good. One of the most serious potential complications of untreated hyperthyroidism is a rapid and irregular heart beat. The most common cause for hyperthyroidism is a condition known as Graves’ disease, in which the immune system produces cells (auto-antibodies) that stimulate the thyroid.

These auto-antibodies mimic TSH, and the thyroid is unable to tell the difference. Even though there are large amounts of T4 being sensed by the pituitary, T3 and T4 production continues to skyrocket, as the thyroid is still being told to make its hormones by this ‘imposter’. Measuring blood levels in this instance reveals very high levels of T3/4, with a very low TSH (the pituitary is attempting to slow production). Signs and symptoms of hyperthyroidism can be thought of as those relating to ‘metabolic overdrive’ (see table above). Certain medications and other diseases such as tumours of the pituitary gland can also cause a hyperthyroid state, however far less commonly.

Thyroid function blood tests

Blood levels of many thyroid hormones can be measured through a blood test. The most useful tests include the following.

Thyroid stimulating hormone (TSH) levels. An abnormality in TSH level indicates thyroid dysfunction, however other tests are needed to determine the specific cause. Free T4 is the test of choice in such circumstances. Should Free T4 be normal, or close to normal, but TSH remains abnormal, then Free T3 should be measured. The problem is that Free T3 testing can be variable and inaccurate.

In years gone by, total T4 and total T3 were measured, however given that the majority of thyroid hormone is bound to protein in the blood, this test of total levels does not really indicate how much of the biologically active (free/unbound) hormone is available.

It is important to note that ‘starvation’ or malnutrition induced hypothyroidism can often reveal a normal, or near normal TSH and T4, however free T3 levels can/will be sub-optimal/low. Remember earlier in the article, we discussed that the problem in such circumstances is often the conversion of T4 to T3, hence these particular results on blood testing. The following table shows some common patterns of thyroid testing findings.

Treatment of thyroid disorders

The treatment of thyroid disorders is dictated by the underlying cause of the dysfunction. Treatments for overactive thyroid includes various medications, irradiation, and removing part/all of the thyroid gland. Treatment for clinically determined and diagnosed underactive thyroid is the administration of thyroxine tablets (T4), which is absorbed and then converted to T3 by the body.

It is common for patients presenting with thyroid disease to undergo a series of tests, including blood tests, unltrasound, biopsy, and nuclear medicine scans. These are performed to not only determine the cause of the dysfunction, but also detect the presence of any cancerous or pre-cancerous changes.

Thyroxine abuse

Being such a powerful metabolic agent, thyroxine is a medication which is used inappropriately by some – everyone from your neighbour who wants to lose a little weight, right through to competitive bodybuilders taking high doses to help them get super-lean. It is often obtained illegally by these groups.

Thyroxine administration is not without significant risk, as these people are effectively playing with their hormone systems, and can induce hyperthyroid states. It is even possible to induce a severely hyperthyroid state known as a ‘thyroid storm’ – a serious condition which can be life-threatening.

Management of diet and over-training induced thyroid dysfunction

Thyroid dysfunction creates a chaotic, disorganised metabolic environment. If any medical intervention is required (tests, medication etc), this needs to be put in place. There is however, a significant amount that can be dome to optimise the metabolic environment through matching nutrition to exercise.

This is where the core principles of Metabolic Precision not only help correct, but also help maintain healthy thyroid function.

Meal construction with the correct proteins

➢ provide a steady supply of the right amino acids to accelerate muscle recovery & adaptations
➢ promotes thermogenesis – the production of heat and maintenance of body temperature

Nutrient-rich plant food choices
➢ provide essential nutrients used in the production of thyroid hormones
➢ we strategically increase volume to prevent the metabolic slowdown which causes plateaus & kills progress

The right fats & Omega-3 ratio
➢ reduce inflammation which contributes to derangement of thyroid and insulin metabolism

Carbohydrates timed and matched to exercise intensity
➢ provides glycogen replenishment and stimulates production of thyroid hormones
➢ prevents catabolic processes and inflammation after intense exercise
➢ increases throughout MP programs as metabolic rate increases

Nutrition Timing & Frequency
➢ optimize hormonal responses to accelerate lean muscle building
➢ eat for health, performance & fat loss all at once

Research-proven resistance training programming
➢ improves thyroid function, insulin function, fat metabolism, and metabolic rate through acquisition of lean muscle
➢ avoid excessively long training sessions reduces the production of cortisol

Metabolically Precise cardio exercise
➢ the right amount promotes increased insulin and thyroid sensitivity
➢ improves the bodies ability to process cortisol and other ‘stress hormones’
➢ promotes fitness & fat loss while avoiding overtraining

The scapegoat?

Many people often blame their thyroid for their lack of results from their chosen exercise and nutrition plan. Yes, genuine cases of primary hypothyroidism do occur, but not as commonly as this excuse. For someone to claim they have an underactive thyroid, without any tests and without diagnosis by a medical practitioner is often an attempt to cover up poor compliance and/or the use of extreme/self destructive nutrition and exercise habits.

Conversely, others will say ‘I’ve tried everything, but I just can’t put on weight, my metabolism/thyroid is too fast’ – this often comes from people looking to increase their muscle mass. The majority of the time (excluding disease), this is simply a result of an individual not eating enough of the correct foods, at the correct times for their goals. Nobody has a metabolism which defies the laws of human physiology!

As with much human physiology, and especially endocrinology, the thyroid is exquisitely complex. To make assumptions about a clients thyroid function without appropriate medical investigation is not only dangerous, but also inappropriate, foolish, and reduces the credibility of the fitness industry. The information presented above is the tip of the iceberg when it comes to thyroid function and endocrinology, and remains one of the most complex areas of science. MP Certified Trainers can refer to section on Thyroid in their Guide to Working With Special Populations.

Members of can download the audio of this article in the MP audio library.


American Association of Clinical Endocrinologists. (2009). Excess thyroid hormone and carbohydrate metabolism. Endocrine Practice, 15(3), 254-62.

Baker, J. S., Graham, M., & Davies, B. (2006). Gym users and abuse of prescription drugs. Journal of the Royal Society of Medicine, 99(7), 331-2.

Boron, W. F., & Boulpaep, E. L. (2009). Medical Physiology: a cellular and molecular approach. Second edition, Philadelphia, Elsevier.
British Medical Journal Best Practice Guidelines (2011). Thyroid function testing.

Gopinath, B., Liew, G., Flood, V. M., Wang, J. J., Kiflev, A., Leeder, S. R., et al (2008). The association between weight gain and thyroid function in an older population. Archives of Internal Medicine, 168(20), 2283-84.

Marras, V., Casini, M. R., Pilia, S., Carta, D., Civolani, P., Porcu, M., et al (2010). Thyroid function in obese children and adolescents. Hormone Research in Paediatrics, 73, 193-7.

Rosenbaum, M., Hirsch, J., Murphy, E., & Leibel, R. L. (2000). Effects of changes in body weight on carbohydrate metabolism, catecholamine excretion, and thyroid function. American Journal of Clinical Nutrition, 71(6), 1421-32.

Special Discounted Offer!!

  • Over 120 NEW Body Sculpting Recipes.
  • Over 300 recipes on-line!
  • My Food Prep video series!
  • Access my Exclusive Members Zone!