Amyotrophic lateral sclerosis (ALS), a progressive neurological disorder, can be challenging to diagnose because its obvious symptoms are common to other neurological conditions.

A thyroid function test can help in a search for evidence of other conditions with shared symptoms and to rule out ALS, which is not associated with impaired thyroid function.

A poorly working thyroid can cause myopathy, or muscle disease, leading to the muscular weakness and cramps also experienced by ALS patients.

How the thyroid gland functions

The thyroid is a small, butterfly-shaped gland located in the lower front of the neck. It plays an essential role in energy metabolism and other processes, like regulating body weight and temperature.

The thyroid produces the thyroxine hormone, also known as T4 because it contains four iodine atoms. In tissues where thyroxine acts, mainly the liver and the brain, the hormone is converted into another hormone called T3.

The amount of thyroxine that the thyroid gland produces is controlled by the pituitary gland.

The pituitary gland controls thyroid function by generating the thyroid stimulating hormone (TSH). The amount of TSH that the pituitary gland releases depends on thyroxine levels in the blood. If thyroxine (T4) levels are low, the pituitary produces a lot of TSH and less of it when levels are high.

What the test measures

The thyroid function test determines whether the gland is working as it should, or if levels are higher than usual (a condition known as hyperthyroidism), or lower than usual (a condition known as hypothyroidism). The term thyroid disease usually refers to hypothyroidism.

Several tests are used to assess thyroid function:

TSH tests

A simple way to detect thyroid abnormalities is to measure TSH levels in the blood. Low TSH levels mean that the thyroid is hyperactive, whereas high levels are a sign of low thyroid function. For an adult, the normal TSH range is between 0.4 and 5.5 mU/mL.

T4 tests

Thyroxine or T4 exists in the blood in two forms. It is either bound to proteins that prevent it from entering its target organs, or it is unbound (free T4). Free T4 is the form that the organs take up and convert to T3. Most tests measure free T4.

The T4 test and the TSH test are usually combined to assess thyroid function. In hyperthyroidism, thyroxine levels are high and TSH levels low. In hypothyroidism, thyroxine levels are low and TSH levels are high. The normal thyroxine range for an adult is between 5  and 11 ug/dL.

T3 tests

T3 tests are usually performed after hyperthyroidism is diagnosed to assess the severity of the condition. In some individuals with hyperthyroidism and low TSH levels, thyroxine levels remain in the normal range. In these cases, high T3 levels can confirm the diagnosis.

Thyroid antibody tests

The immune system normally produces antibodies against foreign substances, such as bacteria, parasites, and viruses. But it can also produce antibodies that stimulate or damage the thyroid. The detection of antibodies against thyroid proteins can help identify the cause of thyroid problems.

One example is the autoimmune disorder Hashimoto’s thyroiditis, where antibodies against thyroid proteins cause hypothyroidism.

Iodine uptake test

Thyroxine contains four iodine atoms, and the thyroid has to pull iodine from the bloodstream to produce this hormone. During an iodine uptake test, the patient swallows a small amount of radioactively labeled iodine, which can be tracked.

The test assesses thyroid function by measuring the amount of radioactive iodine that the thyroid takes up. If the thyroid gathers unusually high amounts of iodine, the gland is hyperactive. If the thyroid takes up small amounts, its activity is low.

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