‘Hypothyroidism’ with normal blood tests
Some patients are convinced that their symptoms of tiredness, weight gain and feeling low are the result of an underactive thyroid gland, even though levels of the hormones thyroxine (T4) and thyroid-stimulating hormone (TSH) are normal.
This mistaken belief has not been helped by numerous articles in newspapers and magazines and inaccurate information on the internet. Unfortunately, a few doctors are prepared to diagnose hypothyroidism and treat patients with thyroid hormones, even though blood tests are normal or with no blood testing at all. These doctors do not usually have any training in thyroid disease.
The following answers to questions frequently asked by patients who feel that they would benefit from thyroid hormone treatment may help to convince you that it is not possible to have an underactive thyroid gland if the blood levels of T4 and TSH are normal.
‘But I have the symptoms of an underactive thyroid…’
The trouble is that the symptoms of an underactive thyroid gland are what we call non-specific. In other words, similar complaints are also made by patients with other problems. For example, many middle-aged women gain weight and this may lead to tiredness, as may the menopause itself, or there may be stress at home and at work. Most of us feel down from time to time and prolonged fatigue might easily result from a recent viral infection. If thyroid blood tests are normal, it makes no sense to insist that the thyroid can still be underactive, rather than consider other diagnoses, changes in lifestyle, or confrontation of the difficulties at home or in the office.
How do you know what level of thyroxine is normal for me?’
In the author’s hospital the normal or reference range for free thyroxine (fT4) in the blood is 10–25 picomoles per litre (pmol/l), although this will vary slightly from laboratory to laboratory. If your free T4 is measured at 14 pmol/l, you might reasonably ask whether it should not be 20 pmol/l and, if so, whether T4 should be given to relieve your symptoms.
The answer lies in the measurement of the pituitary hormone, TSH. By chance, the level of T4 in the blood remains the same from day to day, month to month, and year to year in a healthy person. Any fall in the level is sensed by the pituitary gland, which increases its output of TSH in an attempt to stimulate the thyroid to produce more T4 and return its level to its normal position.
If a normal free T4 of 20 pmol/l fell to a value of 14 pmol/l, which is still within the reference range, the concentration of TSH in the blood would become abnormally high – an indication for considering treatment with thyroxine. If a free T4 of 14 pmol/l is accompanied by a normal TSH concentration, this means that your free T4 concentration is right for you and has been at that level virtually from the day you were born.
Doctors will, however, be suspicious of the combination of a low normal free T4 of, say, 10 pmol/l and a high normal TSH of perhaps 3.2 milliunits per litre (mU/l) (normal is up to 3.5 mU/l), which may indicate that you have underlying autoimmune thyroid disease, especially if thyroid antibodies are present in your blood. Most doctors would treat you with thyroxine, not anticipating any dramatic response, but in order to prevent the onset of more severe hypothyroidism in future years.
‘Why do some patients with normal blood tests feel much better when taking thyroid hormones?’
About 20 per cent of people given a dummy medicine, known as a placebo, believing it to be a real medicine, will feel better no matter what the illness is. This ‘placebo effect’ may last for several weeks or even months before wearing off. If you believe that you might have an underactive thyroid gland despite normal blood tests, any improvement in well-being while taking thyroxine would be the result of your relationship with a ‘sympathetic’ doctor who prescribes what you want. When similar patients were given either placebo or thyroxine for several weeks, not knowing which they were taking, they were unable to tell the difference. In other words, thyroxine was of no more benefit than a dummy tablet in patients who, because of symptoms such as tiredness and weight gain, thought that they had an underactive thyroid gland, although blood tests were normal.
‘What is the harm in taking thyroid hormones if they make me feel better?’
There is no harm for most patients in taking a dose of thyroxine of between 50 and 75 micrograms daily. Unfortunately as the ‘placebo effect’ wears off you may be tempted to take higher and higher doses, which may produce the symptoms of an overactive thyroid gland. This is even more likely to occur if you are taking a combination of thyroxine and triiodothyronine, such as animal thyroid extract (for example, Armour thyroid). Although, in the short term, you may be delighted with any weight loss and apparent increase in energy, in the long term this self-induced hyperthyroidism will lead to osteoporosis and possible fracture and to an irregular heartbeat (atrial fibrillation), heart failure, stroke and even death.
‘I know of some patients who are taking thyroid hormones and steroids because of symptoms like mine’
Addison’s disease occurs when the adrenal glands, which sit on top of the kidneys, fail to produce enough cortisol (hydrocortisone). This occurs from time to time in patients with real hypothyroidism caused by autoimmune disease. There can, however, be no justification whatsoever for doctors prescribing steroids along with thyroid hormones for patients with symptoms of an underactive thyroid gland, but in whom thyroid blood tests have either not been taken or are normal. Steroids should never be prescribed in the belief that the adrenal glands are not working properly without adequate testing beforehand.
KEY POINTS
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Measurements of T4 and TSH are reliable and when taken together allow the doctor to decide when hypothyroidism is present and when it is not present
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It is not possible to have an underactive thyroid gland with unequivocally normal levels of T4 and TSH in the blood
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Thyroid hormone treatment should never be started without confirmatory blood tests