Thyroid Stimulating Hormone (TSH) Test

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The TSH Test

Thyroid disease is very common. Unfortunately thyroid disease is often poorly diagnosed and poorly managed. Many Australians are hypothyroid and remain undiagnosed, perhaps for many years to come.

Thyroid disorders can cause many symptoms which may be misdiagnosed as separate conditions such as depression, atherosclerosis (increased cholesterol), carpal tunnel syndrome, sleep apnoea, anxiety, hypoglycaemia, infertility, insomnia and so on.

Once diagnosed, patients may also not receive optimal thyroid hormone replacement therapy and as a result may continue to suffer unnecessarily. The symptoms experienced by patients with an underactive thyroid are often treated with further drugs or the patients may be referred to specialists for their thyroid related problems and they may even have unnecessary surgery. There are a number of reasons why this occurs and it can be explained as follows:

Many doctors focus solely on test results, in particular the TSH (thyroid stimulating hormone). The TSH test is seen as the golden standard for diagnosis and treatment of hypothyroidism and hyperthyroidism. However the TSH is a hormone released by the pituitary gland in the brain which controls the thyroid gland, but it doesn’t measure thyroid hormone itself and it can therefore be a poor measure of thyroid hormone production.

Typically hypothyroidism is diagnosed when the TSH is high. The thyroid gland can become underactive if the pituitary gland fails to produce enough TSH or if the hypothalamus fails to produce enough TRH (thyrotropin releasing hormone) as seen in the graph below. These conditions can cause a low TSH and hypothyroidism (low thyroid hormone levels). Hypothyroidism cannot be diagnosed with a TSH test if either the pituitary gland or the hypothalamus is not functioning properly.

Thyroid Physiology

 

What is a normal TSH?

In 2002 the US National Academy of Clinical Biochemists (a panel of international experts) published a comprehensive set of guidelines for the Laboratory Support for the Diagnosis and Monitoring of Thyroid Disease. One of the guidelines was to reduce the TSH reference range to 2.5 mIU/L, because the normal reference range for the TSH, used by laboratories, was considered too wide. Numerous large studies like the one below, showed that the most common value for the TSH was 1.25 mIU/L. It was concluded that people who had thyroid antibodies with a TSH of 4 mIU/L or less, who were included in the study, were in the early stages of thyroid disease. 

Drugs and some conditions that decrease or increase the TSH

Certain conditions can also reduce the TSH and give misleading results, such as in depression, pregnancy and in euthyroid sick syndrome (non-thyroidal illness) which is seen in acute or chronic severe illness.

Some of the following drugs/supplements can either increase or decrease the TSH and/or they may interfere with the absorption of thyroxine;

• Amiodarone or Propranolol (heart medications)
• Aspirin
• Warfarin (blood thinning medications)
• Hormone replacement therapy (oestrogen)
• Oral contraceptives
• Anti-depressants, such as Lithium, SSRIs & tricyclic anti-depressants
• Anti-psychotics (Seroquel)
• Metoclopramide
• Morphine
• Aminoglutethimide
• Dopamine
• Anabolic steroids
• Somatostatin analogues
• Non-steroidal anti-inflammatory drugs (NSAIDs)
• Corticosteroids, anti-inflammatory drugs (Prednisolone, dexamethasone)
• Anti-viral medications (Ritonavir)
• Anti-malarial medications (Chloroquine)
• Drugs used to treat diabetes (insulin)
• Drugs used to decrease cholesterol Ion-exchange resins (cholestyramine)
• Drugs used for epilepsy (Phenytoin)
• Drugs used to treat heart failure (Digoxin)
• Iron supplements
• Calcium supplements
• Some antibiotics (Ciprofloxacin, Rifampicin)

Your pharmacist will have a complete list of drugs which may interfere with thyroxine!

A TSH test alone therefore may miss many cases of hypothyroidism. Measuring thyroid hormone levels such as free T4 and free T3 in addition to the TSH is therefore recommended.

Please read this article in conjunction with article Thyroid Function Tests

Copyright © 2011-2015 Monique Atkinson

Disclaimer The information provided is for educational purposes only and is not intended to be medical advice. The contents must not be relied upon in place of advice and treatment from a qualified medical practitioner. THYROID WA SUPPORT GROUP INC. and the author disclaim any liability whatsoever.