UK Flag menu arrow

Select your country

Track my results

TSH Blood Test Meaning: Thyroid Stimulating Hormone Test Explained

Thyroid stimulating hormone or TSH is produced by the pituitary: a pea-sized gland at the bottom of the hypothalamus in the lower part of the brain. The hypothalamus provides an interface between the nervous system and the network of hormone-producing glands around the body called the endocrine system.

Generation of TSH is not the sole function of the pituitary gland: it also releases hormones which control growth, reproduction and the physical response to stress, amongst others. What are hormones? They are chemical compounds which transmit biological signals around the body and control the operation of vital organs.

What is a TSH blood test?

As the name makes clear, thyroid stimulating hormone is produced to trigger another gland within the endocrine system: the thyroid. TSH triggers this double-lobed gland, located in the neck, into producing three additional hormones:

  • Triiodothyronine, also known as T3
  • Thyroxine, aka T4
  • Calcitonin

Calcitonin is used by the body to metabolise the minerals phosphorus and calcium. Meanwhile, T3 and T4, collectively known as the ‘thyroid hormones’, control many, hugely important biological processes. These include:

  • Heat production
  • Metabolism
  • Protein replacement within cells
  • Brain cell growth
  • Bone growth
  • Muscle activation
  • The digestion of proteins, fats and carbohydrates
  • Physical sensitivity to adrenaline

T4 is the primary thyroid hormone, accounting for around 95%. When required, it can be converted into T3, primarily within the brain and liver.

The names T3 and T4 are a reference to the number of iodine atoms in the molecules of each substance. The essential mineral iodine is the primary building block of both hormones. 

What does a TSH blood test look for?

TSH blood tests work via laboratory analysis. Haematology analysers detect cell counts, hormone levels, haemoglobin levels and other constituents of blood in order to assess the patient’s state of health.

What do the results of a TSH test mean?

An individual’s TSH levels will rise and fall according to the amount of T3 and T4 circulating through their bloodstream. If the latter drop, the pituitary will produce more TSH in response. When it comes to blood tests, TSH levels can be used to assess thyroid function.

TSH levels vary from person to person, according to individual physiology and an individual’s health over time. But extensive testing has established a level of 0.5 to 5 mIU/L (milli-international units per litre) as a likely indication of normal thyroid function.

A milli-international unit is one-thousandth of a ‘unit’: an internationally recognised pharmacological measurement, indicating the quantity of a particular biological substance.

What is an overactive thyroid?

It is not all uncommon for thyroid glands to begin to misfire over time, producing either too little or too much T4 and T3. This is an especially common problem in older women, who are no less than 20 times more likely than men to develop thyroid problems.

An overactive thyroid, also known as hyperthyroidism, can trigger heart palpitations, muscle tremors, anxiety, mood swings and hot flushes.

What is an underactive thyroid?

An underactive thyroid, or hypothyroidism, means a person’s thyroid is not producing sufficient triiodothyronine or thyroxine for optimal health. Symptoms associated with hypothyroidism include:

  • Excessive fatigue
  • Aches and pains
  • Feeling unusually sensitive to the cold
  • Weight gain

Can you treat an overactive or underactive thyroid?

Both overactive and underactive thyroid issues can be treated. Prescription medications are available for both conditions, so speak to your GP if you suspect you may have either.

Individuals with hyperthyroidism receive a drug designed to stop excess hormone production. In some cases, a type of radiotherapy is also deployed: this is used to kill off thyroid cells, thereby reducing the gland’s capacity.

Meanwhile, people with an underactive thyroid may be prescribed hormone tablets to boost their sparse blood levels of T3 and T4.

How to take a TSH test at home

If you are concerned about your thyroid health, why not check your TSH levels yourself at home? A painless finger prick blood sample is all it takes. You’ll receive an in-depth professional analysis of your TSH levels and you will then be able to appropriate action if you need to.

Sian Baker

Medically reviewed by Sian Baker, Dip ION mBANT mCNHC – Written by Beth Giddings.

Updated on 1st December 2021