Low Thyroid Test Levels

Hypothyroidism is a prevalent condition and may be the result of underlying conditions, like Hashimoto’s thyroiditis. In order to diagnose an underactive thyroid and isolate the possible cause, a thyroid profile blood test may be requested by your doctor. Apart from confirming diagnosis and providing valuable information as to the possible cause, the thyroid test will also identify the level of low thyroid activity and assist with assessing the efficacy of thyroid drugs.

Thyroid Hormones

The thyroid gland secretes hormones known as T3 (triiodothyronine) and T4 (thyroxine) but its secretion is  controlled by other hormones, mainly those secreted from the hypothalamus and pituitary gland. The hypothalamus secretes a hormone known as thyrotropin-releasing hormone(TRH), which acts on the pituitary gland and this causes the pituitary gland to release thyroid-stimulating hormone (TSH). The thyroid-stimulating hormone (TSH) then travels in the blood to the thyroid gland, and increases the production and secretion of thyroid hormones (T3 and T4). If the thyroid hormones in the blood are too high, then the pituitary gland reduces its secretion of TSH. The entire balance is maintained by a feedback effect, which may be upset if one or more glands or hormones are not operating at the optimum.

Thyroid Blood Test Levels

Your doctor may request any or all of the following :

  • Total or Free T3 level
  • Total or Free T4 level
  • TSH level
  • Thyroid antibodies

Most thyroid hormones, T3 and T4, in the blood is bound to proteins. However a small proportion of these hormones remain unattached to any protein and this is referred to as the free T3 or free T4 level.

Thyroid antibodies are formed in autoimmune diseases, like Hashimoto’s thyroiditis, and the antibodies trigger the body’s immune cells to ‘attack’ the thyroid gland, which causes inflammation and hampers the production and secretion of the thyroid hormones. The thyroid antibody test is not routinely conducted but may be requested based on your doctor’s discretion after the initial thyroid profile results.

It is not advisable to diagnose your thyroid condition based on actual values but rather use it as a guide for future medical consultations with a doctor. Values may fluctuate due to a number of reasons, and age, past medical history and current drug schedule should be taken into consideration before any diagnosis is made. All blood test reports are presented with a reference range, which indicates the normal value range for a specific test. Using the reference range as a guide, you will be able to identify values that are low, normal or high.

Ensure that any reference range that you may be using, which is not on the report, is based on the same measurements as your report. Due to the difference is measurements among different countries, type of test requested, methods or individual labs, as well as the variation in readings due to individual factors,  may determine differences between low, normal or high values in each case.

Mild Hypothyroidism
Secondary Hypothyroidism

Secondary hypothyroidism is a result of low secretion of TSH from the pituitary or the low secretion of thyrotropin-releasing hormone (TRH) from the hypothalamus. A thyrotropin-releasing hormone (TRH) test may have to be done to confirm a reduced TRH secretion.

Under certain circumstances, your doctor may diagnose a condition known as euthyroid sick syndrome (ESS), where the thyroid gland and regulating mechanism is normal but there are low levels of thyroid hormones (T3 and T4).  These low levels may be due to other conditions that are not related to the thyroid gland but the symptoms of low thyroid are usually present. In euthyroid sick syndrome, blood tests will reveal a low T3 and T4 level but a normal TSH level. Even if the TSH level is moderately high or low in euthyroid sick syndrome, it will not be affected to the level recorded in hypothyroidism. Possible causes of euthyroid sick syndrome include fasting or starvation, calorie restriction diets, eating disorders, low protein intake, renal failure, myocardial infarction (heart attack), cirrhosis, diabetic ketoacidosis, sepsis and trauma.


  1. Hypothyroidism. Merck
  2. T3 & T4. Lab Tests Online
  3. Euthyroid Sick Syndrome. Medscape

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