Thyroid-stimulating hormone (TSH) is considered to be the “master controller” of your thyroid. TSH stimulates your thyroid gland to produce the hormones thyroxine (T4) and triiodothyronine (T3), which control how fast your metabolism runs. Read on to find out how TSH works, why doctors order the TSH blood test, and whether your levels are in the normal range.
What is TSH (Thyroid-Stimulating Hormone)?
The “Master Controller” of Your Thyroid
Thyroid-stimulating hormone (TSH), also known as thyrotropin or thyrotrophin, is made by the pituitary gland. The pituitary secretes TSH into the bloodstream, though which it travels to the thyroid gland. The main purpose of TSH is to stimulate the thyroid gland to produce the main thyroid hormones: T3 (triiodothyronine) and T4 (thyroxine) [1, 2].
Since the thyroid needs TSH’s cue to produce hormones, TSH is often viewed as the “master controller” of thyroid function.
The thyroid is important for your overall health because thyroid hormones play major roles in almost every part of the body, including the brain, heart, and liver. They control your metabolism, heart health, mood, and so much more .
Thyroid activity is controlled by the hypothalamus, which produces thyrotropin-releasing hormone (TRH) to tell the pituitary gland to make TSH, thereby causing the thyroid to produce more T3 and T4 when levels are low. If T3 and T4 levels are high, the hypothalamus reduces TRH production and thus, less TSH is made. This process helps keep T3 and T4 in balance .
So even though TSH works as the thyroid’s main switch, T3 and T4 can fine-tune it with the help of the hypothalamus. They do this to prevent their own levels from getting too high (or too low), which maintains balanced thyroid function.
How TSH Works
Thyroid-stimulating hormone increases T3 and T4 levels in several different ways.
First of all, TSH stimulates the production of T3 and T4 by binding to TSH receptors (TSH-R) on the thyroid gland, where it helps the thyroid make more proteins that absorb iodine from the bloodstream. The thyroid then uses iodine to make more T3 and T4 [5, 6].
Iodine can exist in different forms, and it can only be used to make T3 or T4 if it’s in the active form (oxidized). For this reason, TSH further boosts T3 and T4 production by increasing the protein that converts iodine into its active form (thyroid peroxidase) [7, 8].
Thyroid-stimulating hormone also stimulates the release of thyroid hormones into the bloodstream. When T3 and T4 are made, they are stored by binding with another protein (thyroglobulin), which keeps them from entering the bloodstream. TSH helps break down this protein (by increasing proteases), which allows T3 and T4 to be released [7, 9].
TSH Blood Test
Why Doctors Order It
The TSH blood test measures the amount of thyroid-stimulating hormone (TSH) in the blood.
TSH is often the first test doctors use to determine whether you have too little (hypothyroidism) or, conversely, too much thyroid hormones (hyperthyroidism).
High TSH levels can indicate an underactive thyroid, while low TSH levels can indicate an overactive thyroid. This is because when you have an underactive thyroid the thyroid hormones T3 and T4 are low, and so the body responds by increasing TSH to stimulate T3 and T4 production. The opposite happens in the case of an overactive thyroid.
The timing of the TSH test can make a difference in the results because your TSH levels change naturally throughout the day (following a circadian rhythm). Studies show that TSH levels are highest at night and lowest during the afternoon [13, 14, 15, 16].
Your TSH levels also depend on the timing of your meals. Testing levels within 2 hours of a meal instead of in a fasted state can give lower TSH values 
However, looking at TSH levels alone can result in a misdiagnosis. Doctors will usually order a thyroid panel that includes T4 (free T4) and T3 (free or total T3) levels for a more complete and accurate picture .
Normal Levels & Range
TSH is usually measured in uIU/mL (micro-international units per mililiter), which is equivalent to mIU/L (mili-international units per liter). The normal range is 0.4-4.2 mIU/ml, though low-normal and high-normal levels may be suboptimal [19, 20, 21].
Even the normal range may slightly vary from lab to lab. If you already have a thyroid disorder, your doctor may target a narrower TSH range.
TSH normally increases with aging. High levels of TSH are generally associated with negative outcomes in adults and children, but the opposite might not be the case for the elderly: slightly higher TSH levels are considered normal or even protective, whereas lower TSH levels can cause problems [22, 23].
In fact, a number of findings suggest that high TSH levels may be beneficial for living longer. In one study, people with a median age of 98 had significantly higher levels of TSH compared to a control group with a median age of 72 .
In another study of over 2k elderly adults aged 70-79, those with slightly elevated TSH levels had less difficulty walking, were able to walk faster, and were in better shape than those with normal TSH levels. Plus, in a study of 558 adults from ages 85 to 89, those with the highest TSH levels had the lowest overall risk of dying [25, 26].
This post is part of a three-part series about TSH.
- Learn about the causes and symptoms of high TSH and how to lower your levels naturally in this post
- If your TSH is low and you have an overactive thyroid, read about the causes, symptom, and natural fixes in part three
Irregular TSH Levels?
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TSH is a hormone produced by the pituitary that stimulates your thyroid to produce T3 and T4 hormones. Your doctor may order a TSH blood test to assess your thyroid health, usually as part of a thyroid panel.
Normal TSH levels are 0.4-4.2 mIU/ml. If your TSH levels are high, you may have an underactive thyroid. On the other hand, low TSH levels signal an overactive thyroid. TSH levels also normally increase with aging; slightly higher TSH levels are beneficial in older people.