In a nutshell:Hypothyroidism implies an underactive Thyroid gland
Hyperthyyoidism implies an overactive Thyroid gland  
Some brief comments on possible thyroid gland problems(for more detailed information please see further below):
If a patient is having problems with their thyroid therapy then it is important to measure FREE T3 and FREE T4…sometimes rebalancing the free hormone levels makes a large difference to wellbeing.I also consider it important that minerals such as iron, selenium, zinc etc which are important for the transformation of T4 to the much more potent T3 are at optimal levels.
Likewise it is important that there is proper adrenal function - Click here to link to information on the adrenal glands
Brain chemistry and neurotransmitter levels( serotonin, dopamine, GABA etc) are also important and it is possible to measure them quite accurately using a urinary sample.Click here for more information on neurotansmitters and brain chemistry.
Thyroid physiology - a synopsis
The thyroid gland is situated in the neck just behind the Adam’s Apple or thyroid cartilage and this gland sets the pace for the whole body’s metabolism.
It is a little like an orchestral conductor giving out the rhythm to the orchestra.
It affects the activity of each and every cell in the body.
Too little thyroid hormone means that the body’s processes all slow down too much with resultant sluggishness, mental fuzziness and weight gain. Conversely too much thyroid speeds the metabolic processes too much resulting in everything running too fast with resultant weight loss, fast heartbeat, insomnia and anxiety.
In the adult female hypothyroidism may result in menstrual irregularities and difficulty conceiving.
Thyroid Hormone
There are two active thyroid hormones T3 and T4 which are carried ‘bound’ in the blood to transport (carrier) proteins.
While attached to the carrier proteins they are inactive. Think of an analogy involving skiers - they cannot ski when they are sitting in the chair-lift - only when they are ‘released’ from the chairlift can they start to ski.
It is important to realise that the at any given time the vast majority of the body’s thyroid hormone (over 90 %)is totally inactive biochemically because it is bound to transport proteins.
Free Thyroid hormones Free T3 and Free T4 (also known as Free Thyroxine)
Only when the T3 and T4 are released from their carrier proteins are they free to do their work. They are then referred to as Free T3 and Free T4.
Much of the time standard laboratory testing only measures TSH and Total T4 ( total includes the portion bound to carrier protein and the free ‘unbound’ portion). Personally I prefer to look at the level of Free T3 and FreeT4.
T3 is several times more potent than T4 but lasts for substantially less time (in medical jargonese we say that T3 ‘has a shorter half-life’). If metabolism is working properly much of the T4 is converted to T3 but this is dependent on many factors including the person’s mineral (especially iron, zinc and selenium) status.
How reliable are thyroid blood tests?
There is substantial controversy about this issue even in the medical community. It varies from those individuals in the medical community who believe that once TSH and T4 level are normal then there is ‘no problem’ with an individual’s thyroid gland to those (thankfully) few doctors who if twenty people all complaining of tiredness lasting over a year or two were lined up in their waiting room would be very likely to ‘diagnose’ an under-active thyroid gland in the vast majority of them and promptly proceed to dish out replacement thyroid hormone to them. These are two extreme viewpoints and I think that the truth probably lies somewhere in between.
My view is that there is a sub-group of patients who despite having ‘normal’ T4 and TSH may well have a thyroid problem.
This being said there is a potential for real harm to patients if this deviation from normal medical practice is undertaken in a casual and thoughtless manner.
If I am seriously considering a diagnosis of hypothyroidism in the presence of ‘normal’ T4 and TSH (both of which investigations will usually have been carried out long before the patient arrives at my door) the first thing I do is to look at free T3 and free T4 levels along with another TSH and usually at thyroid antibody levels. This means I am measuring how much thyroid hormone is active rather than merely present. If the free T3 and T4 levels are in the lower one-third of the range and the basal body temperature is low then I may begin to seriously entertain the diagnosis. I do not consider starting someone who is tired on thyroid replacement is an appropriate first line of treatment.
Broda Barnes and Basal Body Temperature measurement and hypothyroidism.
An endocrinologist in the United States some forty or so years ago felt that the most reliable and sensitive test for hypothyroidism was the basal body temperature test. This involves measuring the axillary (underarm) temperature first thing in the morning on awakening before there is any physical activity. An old-fashioned mercury thermometer should be used and it should be left in place for ten minutes. I ask patients to take at least ten readings scattered over the course of a month or so. Personally I interpret the readings in the light of the clinical symptoms, total history and blood tests. Good medicine is about collating all the data and not getting diagnostically ‘carried away’ by any particular part of it. We need clinical balance. I am concerned about the hazards of over-treatment of people who do NOT need extra thyroid hormone being given it as much as I have sympathy for those people who may benefit from thyroid supplementation but who are denied it without due consideration.
What is Reverse T3?
Reverse T3 is a form of T3 which has exactly the same chemical composition as ‘normal’ T3 but which does not function to stimulate cells in the way which T3 normally should do. Unfortunately, as well as not functioning to stimulate cells as it normally should, reverse T3 still functions to block receptor sites for ‘normal’ T3 and also still functions in negative feedback by duping the hypothalamus into believing that it is ‘real, functioning T3′ and so reduces the TSH released by the pituitary which, in turn, contrives to reduce the overall amount of thyroid hormone in the body.
Armour Thyroid, Tertroxin (a form of T3), l-thyroxine (or eltroxin).
These are all forms of thyroid replacement therapy. ‘Thyroxine’ is T4 only. ‘Tertroxin’ is the UK brand name of a preparation which contains solely T3. ‘Armour thyroid’ is porcine-derived glandular thyroid which contains mostly T4 but a small amount of T3. Some patients do well on ‘ordinary, inexpensive thyroxine’ while other patients appear to experience side effects on l-thyroxine which they do not experience on Armour thyroid. Some people do better on standard eltroxin with a small amount of added T3. It’s about individual variation in response to treatment and patient preference. I should point out that Armour thyroid is more expensive than normal thyroid hormone replacement.
What about the interaction between the adrenal gland and the thyroid gland?
The adrenal glands are two small glands lying on top of the kidneys( the adrenal-literally ‘attached to the kidneys’ glands). These glands produce cortisol, DHEA and also some of the body’s sex steroid hormones. Adequate amounts of cortisol are important for well-being and to maintain the body in a state of physiological balance. If the adrenal glands become completely defunct we die. If the adrenal glands become so weak that they can produce hardly any cortisol a condition which is described as Addison’s disease. There is also (in my opinion) a group of patients who , while their condition is not severe enough to satisfy the diagnostic criteria for Addison’s disease, nevertheless cannot produce adequate amounts of cortisol. These people may have cold hands and feet , postural hypotension, general weakness and may need to eat very frequently etc. ‘Low’ adrenal function may also be a cause of low basal body temperature.
How can adrenl gland function be tested?
There is a reasonable correlation between the amount of cortisol in the saliva and the blood cortisol. This test is useful as an adjunct alongside the standard medical tests for adrenal function. Salivary cortisol is measured by the patient using a home kit at 8AM, midday, 4PM and midnight. Many individuals who have low adrenal function may be very tired in the morning (as may be a lot of hypothyroid patients) and take a long time to ‘get going’.
Patients with co-existent low thyroid function and low adrenal function.
Patients who have both low thyroid function and low adrenal function may actually feel a lot worse when they are commenced on thyroid replacement therapy. I have certainly experienced this with one or two patients over the years. These patients may remark something along the lines of : ‘I was started on thyroxine / armour thyroid / and I immediately felt worse and began to have palpitations and feel weak and so I stopped the thyroid medication.
There is currently a vogue among a small group of doctors for routinely giving adrenal hormones along with thyroid hormones. How sensible is this? As a general rule it is probably not medically sensible to to decide too quickly that a patient should have cortisol replacement. It is important to at least check salivary cortisol levels if a diagnosis of hypoadrenalism is being seriously entertained.
What is the bottom line about deciding whether thyroid hormone supplementation in the presence of normal basic screening blood tests is worth trying?
Before considering thyroid hormone supplementation in the presence of normal screening blood tests there should be a consistently low basal body temperature and screening tests for minerals that affect thyroid function should be used. If there are symptoms such as irritable bowel syndrome which may denote a low-grade bowel infection then this possibility should be dealt with. Under-active adrenal glands need to be considered. Depending on context oestrogen and progesterone imbalances may also need to be considered. Sometimes when other issues such as these are dealt with the patient’s sense of well-being returns without any need to commit them to long-term thyroid replacement therapy.
The Thyroid is critical to normal growth and development of infants and children
In infants and children the processes of growth and development - especially the development of the nervous system are dependent on normal thyroid function and so all newborns are screened in the maternity units for this.
Thyroid Stimulating Hormone (TSH) controls the level of thyroid hormone in the blood.
The manufacture of thyroid hormone by the thyroid gland is under the control of a part of the brain known as the anterior pituitary.
The anterior pituitary produces a substance called TSH (Thyroid
Stimulating Hormone) which travels to the thyroid from the anterior pituitary via the bloodstream.
However the amount of TSH released is also controlled by another centre in the brain.
The release of TSH is controlled by centres higher up the endocrine command chain in another part of the brain called the hypothalamus The hypothalamus releases TSH-Inhibitory Factor and TSH-Releasing Factor .
The story is further complicated by the fact that TSH manufacture and release from the pituitary is either increased by Thyrotropin Releasing Hormone Releasing Factor (TRH-RF) or else reduced by Thyrotropin Releasing Hormone Inhibitory Factor (TRH-IF).
You could think of TRH-RF as a green light and TRH-IF as a red light to pituitary to release TSH or not.
These substances the travel to the pituitary and either increase or reduce the release of TSH into the bloodstream.
The TSH then travels from the brain to the thryoid gland via the bloodsteam and in turn stimulates the manufacture and release of the thyroid hormones into the bloodstream from there - there is certainly some nifty design work there in Nature!
There is a ‘negative feedback’ control loop in that the hypothamalus continuously monitors the amount of thyroid hormone in the bloodstream. Depending on whether levels of thyroid hormone are are high or low it will then release either TSH-RF ( the ‘green’ light) or TSH-IF (the ‘red’ light) which will then either increase or inhibit TSH release and so the level of thyroid hormone in the bloodstream is continuously monitored.