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What Are Normal Thyroid Levels? Normal TSH Levels & More

The thyroid is an important gland found in all vertebrates (creatures with a backbone). In humans it is located at the front of the neck, close to the Adam’s apple and windpipe. The thyroid consists of two connected lobes that resemble a butterfly in shape. It forms part of the endocrine system, a network of glands that secrete molecular substances called hormones directly into the bloodstream. These transmit biological signals and regulate the function of organs around the body.

The thyroid secretes three key hormones:

  1. Triiodothyronine (T3)
  2. Thyroxine (T4)
  3. Calcitonin

The latter is a peptide hormone. Peptides are short chain amino acids: an organic compound with multiple functions. Calcitonin is involved in the metabolisation of the minerals calcium and phosphorus.

Triiodothyronine and thyroxine contain iodine and are known as the thyroid hormones.

What are the thyroid hormones?

Iodine is a chemical element and a mineral essential to human health: in fact, the densest. It is used by the human body specifically to synthesise the thyroid hormones. Triiodothyronine is often referred to as T3 because it contains three atoms of iodine in each molecule, while thyroxine is called T4 because – you guessed it – each molecule contains four iodine atoms.

Thyroxine or T4 accounts for the great majority of thyroid hormone in the human bloodstream: about 95%. When required, triiodothyronine or T3 levels can be raised by converting T4. This takes place primarily in the liver and brain.

T3 and T4 have multiple, intricate roles within the body and are in fact central to the healthy function of every cell in our body. They are involved in:

  • The production of new proteins within cells.
  • Protein, fat, vitamin and carbohydrate metabolisation.
  • The generation of heat.
  • The growth of neurons (brain cells).
  • Controlling our metabolic rate (the speed of the chemical processes which maintain life).
  • Bone growth.
  • Increasing our body’s response to neurotransmitters like adrenaline.
  • Digestion and weight.
  • Muscle control.

A iodine deficiency means thyroid hormone levels will fall. Typical warning signs of an iodine deficiency include:

  • Excessive sensitivity to cold.
  • Dry skin.
  • Muscle weakness.
  • Excessive fatigue.
  • Disproportionate weight gain.
  • Disportionate blood cholesterol levels.

Low levels of iodine can also cause the thyroid to swell, a condition known as a goiter.

There are number of risk factors for iodine deficiency:

  1. An inadequate diet
  2. Pregnancy
  3. Autoimmune conditions
  4. High blood pressure

Thyroid problems are a related and very common condition. Remarkably, women are 20 times more likely to experience these than men. An overactive thyroid gland is known as hyperthyroidism, and an underactive one as hypothyroidism. The former can cause tremors, palpitations and unusual sensitivity to heat. By contrast, typical hypotheroid symptoms include aching muscles, fatigue and unusual sensitivity to the cold.

While most cases occur in adults, children can also be affected.

How can blood tests detect thyroid problems?

Three key tests are normally conducted to establish an individual’s thyroid hormone levels. These may be combined for the fullest possible picture.

TSH tests

These tests are actually designed to detect a different hormone: thyroid stimulating hormone or TSH. This is produced by the pituitary gland, another multifunctional component of the endocrine system located in the lower part of the brain.

As the name suggests, TSH triggers the production of T4 and T3 by the thyroid gland. Levels of TSH in a person’s bloodstream fluctuate according to the levels of T3 and T4. Low levels will stimulate the release of more TSH, so the quantity present indicates the levels of thyroid hormones in circulation.

T3 tests

Unusually high levels of T3 typically indicate hyperthyroidism and the related condition Graves’ disease – and the higher the level, the more severe the case. 

Meanwhile, low levels of T3 can be caused by some medications (in particular steroids) as well as some illnesses.

T4 tests

Blood levels of T4 can be affected by both your general state of health and by illness. Liver disease, pregnancy and hepatitis C infections can all raise T4 levels. Contraceptive pills can have a similar effect. Conversely, medication containing anabolic steroids can lower T4 levels.

What’s a healthy thyroid level?

There is no one answer to this question, as normal thyroid levels vary by individual. However an indication of average healthy levels can be established by testing a significant number of individuals in order to establish a reference range.

Such testing suggests the following thyroid normal ranges for adults:

  • TSH: 0.5 to 5 mIU/L (milli-international units per litre)
  • T4: 5.0 to 12.0μg/dL (micrograms per decilitre)
  • T3: 80-220 ng/dL (nanograms per decilitre)

What to do if thyroid levels are abnormal

Both hyperthyroidism and hypothyroidism are normally permanent conditions but abnormal thyroid hormone levels can be treated. They are also subject to fluctuating hormone levels over time.

Low levels can be treated with oral medication containing a synthetic thyroid hormone. This has the welcome side effect of also lowering cholesterol which often rises in hypothyroidism patients. Your GP is likely to check your hormone levels on a yearly basis in order to ensure you are taking the optimal dose.

Similarly, alternative medications can lower thyroid hormone levels in hyperthyroid patients. Treatment for this condition sometimes also involves surgery.

How thyroid tests can help manage disorders

A home administered thyroid function test is a good place to start if you suspect you may have a thyroid issue. Send in a simple finger prick sample and receive a detailed laboratory analysis of your current hormone levels. This will enable you to take control of your health and seek appropriate medical treatment.

Sian Baker

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

Updated on 1st December 2021