What is Thyroid Stimulating Hormone (TSH) and What Do My Results Mean?

Katie Crino
Katie Crino
January 27, 2023 ยท 3 min read Sources Verified

What is TSH?

If you've ever gotten a hormone or fertility test, you may be familiar with your thyroid stimulating hormone (more commonly referenced as TSH). Before you can understand what exactly TSH does, you need to understand your thyroid. Your thyroid is a small gland in front of your neck that resembles a butterfly opening its wings. Think of your thyroid as an army general. It produces and controls armies of hormones that go out into your body and control and regulate nearly every organ. Your thyroid hormones affect your muscles, mood, weight, and more. Too much thyroid hormone can speed up your body's organs and processes, while too little can slow them down. Therefore, our body has a way of regulating this.

TSH and thyroid hormone cycle
How Your Thyroid Works

Our body regulates the production of thyroid hormones with our pituitary gland, a different gland located in our brain. This gland controls your thyroid by creating TSH. TSH then regulates your thyroid, telling it how much hormone it should make and release. If your thyroid hormone levels are too low, more TSH is made to tell your thyroid to produce more hormones. On the contrary, if levels are too high, less TSH is made, and therefore, fewer thyroid hormones are produced.

What are T3 and T4 hormones?

Your T3 and T4 hormones are the hormones produced by your thyroid gland. As mentioned before, these hormones play an important role in regulating your body. Both T3 and T4 come in two forms, bound and free. Bound T3 and T4 are attached to proteins, while free T3 and T4 are not attached to anything. Typically, these hormones are tested in conjunction with TSH when testing for hypo- or hyperthyroidism. More often, doctors will test for free T4 over total T4 because it's believed to be a more accurate representation, while total T3 tests are more common than free T3 tests.

What do my TSH results mean?

Your TSH levels are tested to see the functionality of your thyroid. The normal healthy range for TSH levels is 0.4-4.5 mIU/L and typically indicates a functional thyroid. Results below 0.4 mIU/L are signs of an overactive thyroid, while results above 4.5 mIU/L are signs of an underactive thyroid. However, T3 and T4 tests are required to diagnose hypo- or hyperthyroidism further.

Potential Condition TSH Levels T3/T4 Levels
Subclinical Hypothyroidism Above 4.5 mIU/L Normal T4 Levels
Overt Hypothyroidism Above 4.5 mIU/L Low T4 Levels
Subclinical Hyperthyroidism Below 0.4 mIU/L Normal T3/T4 Levels
Overt Hyperthyroidism Below 0.1 mIU/L Elevated T3/T4 Levels
Rugge, JB. et al. NCBI, 2014

What if my TSH levels are above 4.5 mIU/L?

Subclinical hypothyroidism (a milder form of hypothyroidism) could be indicated by TSH levels above 4.5 mIU/L with normal T4 levels. Subclinical hypothyroidism can progress into overt hypothyroidism and happens in about 2-5% of cases yearly. However, even with normal T4 levels, subclinical hypothyroidism is usually treated if your TSH levels are above 10 mIU/L or if you are pregnant or contemplating pregnancy. That said, if you are neither of these cases, there is controversy on whether subclinical hypothyroidism needs treatment or not. It's important to talk to your doctor about elevated TSH levels or low T4 or T3 levels.

How your thyroid functions in Hypothyroidism
How Hypothyroidism Works

Overt hypothyroidism results from an underactive thyroid and means your body is not making enough thyroid hormones (T3 and T4). This results in the slowing of your body's functions. If left untreated, it can contribute to high cholesterol and, in rare situations, can lead to a myxedema coma (when your body's functions slow severely) and can be life-threatening.

Overt hypothyroidism could be indicated by TSH levels above 4.5 mIU/L and T4 levels lower than normal. Hypothyroidism is more likely to develop in women and those over 60. It is also more commonly developed in those with Turner syndrome, who were pregnant in the last six months, who have a family history of thyroid disease, who received radiation treatment in the neck or chest area, or who have previously struggled with a thyroid problem. It's important to treat hypothyroidism during pregnancy as it can affect both the mother and child.

Some signs of hypothyroidism include fatigue, muscle pain, thinning and dry hair, irregular menstrual periods, and more. Certain medications and additional testing are used to treat hypothyroidism.

What if my TSH levels are below 0.4 mIU/L?

Hyperthyroidism results from an overactive thyroid that produces excessive thyroid hormones (T3 and T4). It is present in 1 in every 100 Americans and can lead to serious health issues like strokes, osteoporosis, eye disease, and more. Like hypothyroidism, there is both subclinical and overt hyperthyroidism.

How your thyroid functions in Hyperthyroidism
How Hyperthyroidism Works

Subclinical hyperthyroidism is characterized by TSH levels below 0.4 mIU/L and normal levels of T4 and T3. People with subclinical hyperthyroidism usually don't have the same symptoms and signs as overt hyperthyroidism. They are also less likely to progress from subclinical to overt hyperthyroidism than those with subclinical hypothyroidism. It's important to talk to your doctor about low TSH or elevated T4 or T3 levels.

Overt hyperthyroidism is characterized by TSH levels below 0.1 mIU/L and elevated T4 or T3 levels. Hyperthyroidism is more common in women and those over 60. It can also be more likely if you consume high amounts of iodine, use nicotine, have a family history of thyroid disease, have other health-related issues, or were pregnant in the last six months. Hyperthyroidism can be a problem during pregnancy if it becomes severe. Overt hyperthyroidism can be treated with medications, surgery, or radioiodine therapy.

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