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When you think of hypothyroidism, you usually think of low levels of thyroid hormones like thyroxine (T4) and triiodothyronine (T3). But, another hormone plays a crucial role in regulating the production and secretion of your thyroid hormone: thyroid-stimulating hormone (TSH).
When the pituitary gland releases TSH, it tells your thyroid to make and release thyroid hormone. As thyroid hormone levels increase, your pituitary gland slows down and stops releasing TSH. Thyroid hormone levels decline as your cells use this hormone. The decline signals your pituitary gland to release more TSH, and the cycle continues.
People with an underactive thyroid typically have high TSH levels with low T4 and T3 levels. In hypothyroidism, the pituitary gland tries to stimulate your thyroid to make thyroid hormones by releasing more and more TSH. But for various reasons, it can’t.
When your TSH is below the low end of the reference range – or TSH is undetectable – due to medication, this is called “TSH suppression.” It’s somewhat common for patients with hypothyroidism to have suppressed TSH levels, as in some people, it helps with symptom management. Yet, there are risks associated with too low a TSH level. Ahead, we discuss TSH suppression and its implications on thyroid disease and your health.
The American Thyroid Association hypothyroidism guidelines consider a normal TSH range as 0.4–4.0 mIU/L. There is controversy about adjusting this range based on age, with a more lenient range for older adults. But that is a story for another day!
Suppression is defined as a level below the reference range, and there are several different levels of TSH suppression:
- Mild suppression: 0.1 to 0.5 mU/L
- Moderate suppression: 0.1 to 0.01 mU/L
- Severe suppression: below 0.01 mU/L
The duration and extent of TSH suppression depend on the goal.
Levothyroxine is a synthetic version of thyroxine (T4). NDT is a natural drug containing T4 and the active thyroid hormone triiodothyronine (T3). When you take these medications, thyroid levels in your blood increase, signaling your pituitary gland to stop releasing TSH.
Over time and with consistent dosing, your TSH level will drop. Increasing your thyroid hormone replacement medication dose will further reduce your TSH level. Most people with moderate to severely suppressed TSH levels take higher doses of thyroid medication.
Two groups can benefit from TSH suppression: those with thyroid cancer or hypothyroidism.
Thyroid cancer patient
- Those with an active tumor
- Those with a very aggressive form of thyroid cancer, despite radioactive iodine and surgery
In these situations, doctors target a moderately suppressed initial TSH level of less than 0.1 mU/L. This goal can change over time based on other risk factors.
For low-risk thyroid cancer patients, guidelines recommend aiming for a TSH level around 0.1–0.5 mU/L. Going lower may increase risks associated with TSH suppression without gaining more benefits.
Regardless of the cause, patients with hypothyroidism are commonly prescribed thyroid hormone replacement drugs to help manage thyroid hormone levels. Since these drugs replace your natural thyroid production, it signals the pituitary gland to stop producing TSH.
Suppressing your TSH too much can have negative consequences on your health. Some describe this situation as “subclinical thyrotoxicosis,” a mild form of hyperthyroidism.
At normal range TSH levels, bones are protected. TSH-suppressive therapy, however, may lead to bone loss, causing an increased risk of
- Bone fractures
Post-menopausal women are at a higher risk of fracture and bone-related complications associated with TSH suppression compared to men and pre-menopausal women. Post-menopausal women currently on or starting TSH suppression therapy are at a higher risk of osteoporosis and should have their bone density checked yearly. If needed, your healthcare provider can prescribe anti-reabsorption therapy such as Fosamax, Reclast, or Actonel to help prevent bone loss.
TSH suppression can also affect your heart. High thyroid hormone levels may stimulate the muscles in your heart, changing how it functions.
A 2022 review showed those on TSH-suppressing doses of thyroid hormone replacement drugs after thyroid surgery had an increased risk of cardiovascular disease, specifically atrial fibrillation. In this condition, your heart rhythm is irregular. Also, those receiving TSH suppression had faster heartbeats and higher blood pressure than those not undergoing TSH suppression.
Apart from the effects on your bones and heart, other symptoms caused by suppressed low TSH include
As discussed, TSH suppression is a natural consequence of taking higher doses of thyroid hormone replacement medication. However, suppressions put you at a greater risk of developing signs and symptoms of subclinical thyrotoxicosis.
If you are wondering if your hypothyroidism treatment regimen is right for you, schedule a free, no-obligation appointment with our care managers. We have a team of thyroid specialists that take a personalized approach to managing your hypothyroidism.
Also, please note that while TSH suppression is commonly used to treat thyroid cancer, our thyroid providers do not see patients with a history of thyroid cancer.