Understanding thyroid testing

Problems with the thyroid affect many people, are more commonly encountered among women and increase with age. The American Thyroid Association reports that 12% of people will develop thyroid problems during their lifetime. Figures in the UK are likely to be similar to those in the United States.

The thyroid controls multiple areas of our body. These include three main areas.

  1. The brain and hormonal system.
  2. The gastrointestinal tract, liver and gallbladder.
  3. Cholesterol, fat metabolism and cardiovascular health.

Symptoms of hypothyroid

Thyroid function can deteriorate for a number of reasons. Firstly the thyroid gland itself may fail due to an autoimmune attack on the thyroid. This is called Hashimoto's thyroiditis. Secondly the pituitary gland may stop stimulating the thyroid properly, thirdly the proteins that transport thyroid hormone may fail to work correctly and finally enzymes called deiodinases, that convert the thyroid hormone T4 into its more active counterpart T3, may malfunction. Hypothyroid or low activity of the thyroid gland leads to a number of symptoms the most common of which are:

  • Fatigue, depression and brain fog.
  • Cold hands and feet.
  • Weight gain and difficulty losing weight.
  • Loss of hair - particularly from the eyebrows and dry flaky skin.
  • Facial swelling and reduced fertility.
  • Constipation
  • Increased cardiovascular risk factors especially number of LDL cholesterol particles. There is also an increased risk of oxidised LDL particles which create plaque deposits in the arteries more easily.

How the thyroid works

The thyroid is wrapped around the front of our windpipe in the neck. It releases thyroid hormones triiodothyronin - T3 and thyroxine - T4, which it makes from the amino acid tyrosine. How much it produces is controlled by a structure at the base of the brain called the pituitary gland. The pituitary produces thyroid stimulating hormone (TSH), sometimes called thyrotropin that signals the thyroid to produce more thyroid hormone. The pituitary in turn is stimulated by the hypothalamus, which produces thyrotropin releasing hormone (TRH). The TRH from the hypothalamus stimulates TSH release from the pituitary, which in turn drives the thyroid to produce thyroid hormone.

The thyroid hormone is mostly produced as T4. This is converted to T3 in the tissues and it is this T3 that is the active form of thyroid hormone. An additional factor to be aware of is that these T3 and T4 hormones are fat soluble, but not very soluble in blood, meaning that they get transported in blood bound to the transport proteins, thyroxine binding globulin (TBG), transthyretin and albumin.

With the above explanation you will hopefully be able to understand the results you get from tests carried out by your doctor or local nutritional practice. It will also help you understand some of the ways in which thyroid problems are often underdiagnosed when only a few of your thyroid parameters are measured.

Normal laboratory ranges for thyroid parameters

Laboratory standard values vary between labs depending on a number of factors. In the case of thyroid parameters there is plenty of debate around what constitutes a hypothyroid state. Listed below are some of the standard values you will often find, starting with the lower limit and ending with the upper limit.

  • Thyroxine or T4 is the main hormone released by the thyroid. 4.5 - 11.5 µg/dl
  • Free T4 is the T4 that is not bound to TBG 0.8 - 2.8 ng/dl
  • Triiodothyronine or T3 is the active hormone. 75 - 200 ng/dl
  • Free T3 is the T3 that is not bound to TBG 2.3 - 4.2 pg/ml
  • Reverse T3 (rT3) is T3 that has not been converted properly
  • TSH is thyroid stimulating hormone from the pituitary 0.5 - 4.7 mIU/L
  • Levels of thyroid antibodies and TBG can also be measured.

Note that the most routine thyroid test done by the NHS is for T4 and TSH. Other parameters are normally ignored unless specific conditions are suspected. There are a number of issues that can cause misdiagnosis:

  1. The TSH levels at which thyroid problems are diagnosed are controversial. The standard NHS upper limit for TSH is around 4.5 mIU/L. However this figure is based on an average of people including those with faulty thyroid function. In other words it is likely to be too high. When all people with faulty thyroid function have been excluded the correct upper limit for TSH is between 2.2 and 2.5 mIU/L. If your value is above 2.5 it is a good idea to get more parameters measured to check if there is a problem.*
  2. The most common issue that arises is the diagnosis of Hashimoto’s disease (an autoimmune attack on the thyroid), which is normally only diagnosed when there is a high level of TSH and a low level of T4, a situation that normally only affects between 1 in 500 to 1 in 1,000 people. However, there is good evidence that before TSH levels are raised hypothyroid states can be detected in more than 1 in 20 people. These people often experience the symptoms of hypothyroidism, but their doctors tell them nothing is wrong.
  3. In the elderly there is a tendency for the TSH levels to rise, especially over the age of 80. As such the TSH levels at which treatment may be considered can also be expected to rise.

*TSH levels can vary by as much as 50% over the course of a day, so follow up measurements may still be a good idea if you are experiencing hypothyroid symptoms and your TSH measurement is above 2.0 for instance. Equally a TSH measurement of 3.0 may be considered high, but if a 2nd measurement is 2.0 then there may well be no problem.

Conditions that cause hypothyroid symptoms

Problems related to the thyroid can occur for a number of reasons. Reason number 1 is the one that is most often detected and treated by the NHS.

  1. Thyroid problem. Sometimes there is a problem with the thyroid gland itself. Most cases involve the immune system attacking the thyroid gland, which is called Hashimoto's thyroiditis. In this case a patient will present with a low T4 level and a high TSH reading indicating that the pituitary gland has detected low levels of thyroid hormone and is releasing thyroid stimulating hormone to raise them again. Levels of antibodies to thyroid peroxidase (TPO) and thyroglobulin (TGB) are often raised with Hashimoto's, but not in all cases.
  2. Conversion problem. Another common problem is that thyroid hormone – T4 is not being converted to its active form T3 very well. In this instance patients present with normal levels of T4 and TSH, but low levels of T3. This type of hypothyroidism can be caused by nutrient deficiencies, high levels of stress and conditions such as Crohn’s disease and chronic fatigue syndrome.
  3. Pituitary problem. In other harder to diagnose cases there may be a problem with the pituitary gland and its response to unbalanced blood sugar levels. In these cases the stress hormone cortisol is normally raised. People with this problem usually present with TSH levels less than 2.0mIU /L and a low, but in range T4 level.
  4. Thyroid resistance. In some cases patients are resistant to thyroid hormone, in much the same way that type 2 diabetics are normally resistant to insulin. Thyroid hormone can’t affect the cells either because they don’t contain enough thyroid receptors or the receptors don’t work properly. This is a tricky scenario as there is no way to easily measure thyroid receptors and all the standard thyroid parameters can be normal. This situation can arise either due to a patient’s genetics or elevated homocysteine which may reflect long term smoking, drinking, a junk food diet or stress.
  5. Binding protein problem. The proteins that carry thyroid hormone in the blood can in some cases be too numerous or too rare. This can be measured along with other standard thyroid parameters.
    1. If oestrogen levels are too high, perhaps due to birth control pills or hormone replacement therapy then the thyroid binding proteins can become too numerous leading to normal TSH and T4, but low levels of T3.
    2. If testosterone levels are too high (in women with PCOS and/or insulin resistance) then thyroid binding hormone can be too low with a normal TSH and T4 level, but high levels of T3 can result.

For all the 5 types of condition mentioned above there are nutritional and lifestyle changes that can be made to address the underlying problem. While treatment with thyroid hormone may improve symptoms and risk factors, it does not resolve the underlying reason for your thyroid problem. In many instances you may be better off making changes to your diet and lifestyle. Please consult your local nutritional therapist for more advice on what you should do.