Thyroid Monitoring

The measurement of TSH in a blood sample is used as part of the monitoring of thyroid replacement therapy . TSH is released by the pituitary gland and then via the blood reaches the thyroid where it signals the thyroid to release thyroid hormones T4 and T3. The amount released of these two thyroid hormones depends on the level of TSH, the higher the level of TSH then the higher the amount of T4/T3 released. In reverse the hormones T4/T3 feed back via the blood stream to the pituitary altering the output of TSH, high T4/T3 switching down the output of TSH, low T4/T3 switching up the TSH release. Obviously if the thyroid is not working (hypothyroidism) then the TSH is very high whereas if the thyroid is overactive as in thyrotoxicosis then the TSH is suppressed (e.g. undetectable). Likewise if the patient is having too little thyroid hormone replacement TSH is elevated whereas if having too much TSH is suppressed.

The TSH value is given along with values for the 'normal' population. It is important to understand how this is derived. The usual method is to take many hundreds of people who are not known to have thyroid problems and to measure their TSH. In our area, 95 out of every 100 people (95%) have a TSH between 0.4 and 4.0 mU/L and this is known as the 'reference interval'. This means that 5 people in every 100 will have a TSH less that 0.4 or above 4.0mU/L and yet be normal for them. Nevertheless a TSH which is below 0.4mU/L (called suppressed ), especially when substantially so (e.g. less than 0.1 mU/L or undetectable i.e. below 0.03 mU/L), is considered outside the reference range and may be considered unusual for most people and worthy of investigation.

When monitoring thyroid replacement difficulties do arise on assessing the correct dosage when TSH is suppressed. Research has therefore been undertaken to find out if a suppressed TSH maintained for years is harmful.

Recently research reports have highlighted an increased risk of a fast disturbance of heart rhythm (called atrial fibrillation), heart attacks, angina, stroke and fractures in those over the age of 55 years who have a suppressed TSH. For atrial fibrillation and for fractures of the spine and hip, the risk overall appears to be greater if TSH is less that 0.1 mU/L as compared to 0.1 to 0.4 mU/L, whereas for cardiovascular and cerebrovascular risk there is insufficient data to clarify at what level of suppression the risk is worse. Please note that the risk has been studied mainly in women, in those aged over 55years and is unknown for younger age groups. The risk is said to be directly due to the suppression of TSH and not due to the actual thyroxine replacement itself. Please note that most of the people studied in these reports who had a suppressed TSH were NOT on thyroxine replacement therapy and therefore care is required in assuming that a patient taking thyroxine with a suppressed TSH has the same risk as a person who is not on thyroid replacement therapy with a suppressed TSH. As we as yet do not know if there is a difference between these two groups.

Atrial fibrillation

Atrial fibrillation this is an erratic pulse rate which can become very fast with the risk of heart attack or clots thrown up into the brain causing stroke. In one study some 2007 persons aged 60 years or older were followed up for 10 years and it was found that 28% developed atrial fibrillation if TSH was <0.1 mU/L, 16% if TSH was 0.1 to 0.4 mU/L and 11% if TSH was within the normal range. This means that the risk of atrial fibrillation is nearly 3 fold higher if the TSH is suppressed

Reference; Sawin. New England Journal of Medicine, 1992, 327 1451

Circulatory disorders

This report measured circulatory disorders over a 5 year follow up.
For cardiovascular diseases (e.g. heart attacks, angina, peripheral vascular disorders) there was a 2.2 fold higher risk in those with TSH less than 0.5mU/L
For cerebrovascular diseases ( e.g. stroke) the risk was 2.8 fold higher in those with TSH less than 0.5 mU/L.

Reference; Parle et al Lancet 2001 Volume 358; pages 861 to 865

Fractures

This report measured the risk of fractures of the hip and vertebrae in 686 women aged 65 years or more

Reference; Bauer et al Annals of Internal Medicine, 2001, volume 134, pages 561 to 568

Some patients do require to maintain a suppressed TSH such as those with treated cancer of the thyroid, so as to prevent the original condition from recurring. However others especially those in the age range likely to develop the above complications (e.g. aged 50 years and over) need to discuss their own situation with their medical advisor. Some patients dislike having their thyroxine replacement reduced for symptoms of tiredness etc resume or get worse. The risk of maintaining a higher thyroid replacement dosage needs to be weighed up against the potential (remember it is only potential ) risk of developing the above conditions.

Ó NHS Tayside; 2006; version 1.0

Disclaimer; no liability whatsoever is accepted for information given and all such information, especially with regard to drug usage (UK version provided), must be checked with a person’s health provider.