TSH 101: How Do Low Or High Levels Affect Me?

TSH 101: How Do Low Or High Levels Affect Me?

Thyroid Stimulating Hormone (TSH) readings that are too high or low point to underlying thyroid problems, which can have serious implications on pregnancy and overall health. Here’s what you need to know about TSH levels and what your reading might mean for you. 

Thyroid Stimulating Hormone or TSH for short, plays an important function in governing our overall health. As its name suggests, this hormone affects the thyroid gland by stimulating its activity levels. The more TSH, the more active the thyroid gland becomes, and vice versa. 

Hence, the human body uses TSH as a lever to ensure proper levels of thyroid activity. In healthy individuals, thyroid activity is kept within normal ranges, but changes during pregnancy. 

Here’s a look at what TSH does, what levels are healthy, and what changes could be expected during pregnancy. 


What does TSH do in the body [1]? 

The thyroid, a butterfly-shaped gland in the lower front of the neck, is a part of the human endocrine system. It is responsible for releasing appropriate levels of 2 hormones, triiodothyronine (T3) and thyroxine (T4).

T3 and T4 play a number of critical roles in the body, such as governing metabolism, as well as helping the brain, heart, liver, muscles and other organs work properly. These hormones are also essential for brain and physical development in infants. 

So how does the thyroid know how much T3 and T4 to release? Well, the gland takes its cue from TSH, which is a hormone secreted by the pituitary gland, a pea-sized gland located at the base of your skull.

When the levels of T3 and T4 are low, more TSH is secreted, stimulating the thyroid to release more hormones. Conversely, when T3 and T4 levels are high, the pituitary gland lowers the level of TSH it secretes, which causes the thyroid to reduce the amount of T3 and T4 hormones released. 


What can TSH tests reveal [2]? 

Because of this closely linked relationship, TSH tests are used to find out if the thyroid is functioning as it should, by revealing if your thyroid levels are too high or too low. 

To perform a TSH test, a blood sample is required. This means you will need to have your blood drawn by a phlebotomist or at a medical facility. A small vial of blood is all that is needed for a TSH test, unless other blood tests are also required. 

When interpreting TSH test results, recall that TSH and T3 and T4 thyroid hormones have an inverse relationship.

Therefore, if your TSH levels are found to be high, it means your T3 and T4 levels are low. This is known as hypothyroidism, and symptoms include:

  • Weight gain
  • Tiredness
  • Hair loss
  • Low tolerance for cold temperatures
  • Irregular menstrual periods
  • Constipation

On the other hand, if your TSH levels are found to be low, that means that your T3 and T4 levels are high. This is known as hyperthyroidism, which has the following symptoms:

  • Anxiety
  • Weight loss
  • Tremors in the hands
  • Increased heart rate
  • Puffiness
  • Bulging of the eyes
  • Difficulty sleeping

If you’re suffering from these symptoms, a TSH test can help to make a confirmed diagnosis.

The twoplus Fertility Hormone Test Kit combines 5 hormone tests into 1, including TSH and free T4, giving you a rounded picture of your endocrine health. Simply order online, and our trained phlebotomist will visit you for a quick blood sample collection. 

Normal TSH levels during adulthood and pregnancy [3] 


Normal TSH levels per age group [milliunits per litre, mU/L]


18 to 50: 0.5 - 4.1 mU/L

51 to 70: 0.5 - 4.5 mU/L

71 to 90: 0.4 - 5.2 mU/L

Pregnant women (18 to 45 years old)

First trimester: 0.6 - 3.4 mU/L 

Second trimester: 0.37 - 3.6 mU/L 

Third trimester: 0.38 - 4.0 mU/L 

The table above displays ranges of TSH levels that are considered normal, according to age group, and during pregnancy. 

Readings that fall outside of these ranges at either end are considered low and high, respectively. For example, for a 75-year-old, a reading less than 0.4 mU/L would be considered low, and a reading more than 5.2 mU/L would be considered high. 

For babies and children, vastly different ranges have been observed throughout their growing years. TSH levels tend to start off high in the few days following birth, and gradually decrease and towards adulthood. 


Why do TSH levels change during pregnancy [4]? 

As seen in the table above, changes in TSH levels are seen during pregnancy. During all 3 trimesters, there is a fall in the upper ends of the range; meanwhile, a rise in the lower ends of the range is seen in the first trimester. 

In other words, thyroid hormones increase during pregnancy. 

This is attributed to 2 pregnancy-related hormones — Human Chorionic Gonadotropin (hCG) and oestrogen. hCG, which increases during pregnancy, is similar to TSH and mildly stimulates the thyroid to create more hormones. Meanwhile, increased oestrogen causes higher levels of thyroid-binding globulin, which is a protein that transports thyroid hormones in blood. 

There’s an important reason for the elevation in thyroid hormones during pregnancy. During the first 3 trimesters, a baby depends on its mother’s supply of thyroid hormones, which is critical for proper brain and nervous system development. 

From 12 weeks onwards, the baby’s own thyroid starts to take over, but is only able to make enough hormones for itself at around 18 to 20 weeks. 


Abnormal TSH levels and pregnancy [5] 

While pregnancy brings about slight changes to TSH levels, readings that deviate too much from normal ranges should not be taken lightly, as serious complications can arise.

Hypothyroidism during pregnancy

A TSH reading that is too high indicates an underactive thyroid (hypothyroidism), which can affect your pregnancy in the following ways:

  • Preeclampsia (a dangerous rise in blood pressure during late pregnancy)
  • Anaemia
  • Miscarriage
  • Low birthweight
  • Stillbirth
  • Congestive heart failure (rarely)

What treatments are available for hypothyroidism during pregnancy?

Treating hypothyroidism during pregnancy focuses on elevating the levels of T4 hormones in the body, until the baby’s own thyroid is fully functional at weeks 18 to 20. To this end, a thyroid hormone medicine known as levothyroxine will be prescribed. 

Levothyroxine is the same as the T4 hormones that the thyroid creates. In order to ensure that T4 levels stay within an acceptable range, regular tests will likely be required, with corresponding dosage adjustments. 

Those evaluated to have subclinical hypothyroidism (mild cases with no clear symptoms) may not need to undergo treatment.

Hyperthyroidism during pregnancy

On the other hand, a TSH reading that is too low means your thyroid is overactive (hyperthyroidism), and could affect your pregnancy in the following ways:

  • Miscarriage
  • Premature birth
  • Low birthweight
  • Preeclampsia (a dangerous rise in blood pressure during late pregnancy)
  • Thyroid storm (a sudden, severe worsening of symptoms)
  • Congestive heart failure

What treatments are available for hyperthyroidism during pregnancy?

In treating hyperthyroidism during pregnancy, the goal is to lower excessive amounts of thyroid hormones to a level safe for your baby. This is achieved by prescribing anti-thyroid medications such as propylthiouracil (PTU) and methimazole to cause the thyroid to make less hormones. 

Both of these drugs slightly increase the chances of birth defects, although this is rare. Of the 2 drugs, PTU is considered to be safer, and is prescribed during the first trimester. Thereafter, your doctor may switch to methimazole for continued treatment. These antithyroid medications may be discontinued at the third trimester; some women no longer need them at this stage of the pregnancy.

Subclinical or very mild cases of hyperthyroidism may not require treatment, or may only require management of symptoms such as vomiting and dehydration. 


Conclusion: Tips for a healthy thyroid

Clearly, thyroid health is highly important, not just for pregnant mothers, but for everyone else as well. 

Here are some tips to help keep your thyroid in top shape. 

Get enough iodine in your diet [6] 

Iodine is an essential micronutrient. It is required for the thyroid to produce T3 and T4 hormones, and ensure healthy functioning.

According to the American Thyroid Association, the recommended daily allowance (RDA) of iodine should be 150 micrograms (mcg) for adults. During pregnancy or while breastfeeding, the RDA is 220 mcg and 290 mcg respectively. 

Unlike other micronutrients, the body is unable to make iodine on its own, which means we have to ingest it from food sources. Therefore, getting adequate iodine in your diet is crucial for maintaining thyroid health. 


Avoid environmental toxins [7]

Certain compounds that are found in the environment have been shown to interfere with the human endocrine system, of which the thyroid is a crucial component. 

These are known as endocrine disruptors, and limiting your exposure to them is important in keeping your thyroid (and other hormone glands) healthy. 

Some commonly found endocrine disruptors with known thyroid impacts include:

  • Perfluorinated chemicals: PFCs, found in some carpets, flame-resistant and waterproof clothing, and non-stick cookware.
  • Phthalates: Found in some fragranced products and soft plastics.
  • Bisphenol-A: Found in some hard plastics and canned food linings.
  • Triclosan: Commonly found in antibacterial products.  

Cook your cruciferous vegetables [8]

Kale, broccoli, cauliflower, cabbage and Brussels sprouts are popular vegetables that belong to the cruciferous category. While they are healthy and delicious, eating them raw can interfere with normal thyroid function, especially if you’re deficient in iodine. 

The culprit is a compound known as goitrogens, which can block iodine absorption, disrupt thyroid hormone formation, and even lower TSH levels.

Thankfully, there’s no need to stop eating cruciferous vegetables; cooking greatly diminishes the effects of goitrogens, so be sure to steam, saute, boil or roast those veggies before you eat them.


[1] Endocrineweb, What Are T3, T4, And TSH?,  https://www.endocrineweb.com/thyroid-what-are-t3-t4-tsh
[2] National Library Of Medicine, TSH (Thyroid-stimulating Hormone) Test, https://medlineplus.gov/lab-tests/tsh-thyroid-stimulating-hormone-test/
[3] Healthline, All About Standard TSH Ranges By Age And Life Stage,  https://www.healthline.com/health/tsh-normal-range-by-age
[4] National Endocrine And Metabolic Diseases Information Service, Pregnancy And Thyroid Disease, https://www.niddk.nih.gov/-/media/Files/Endocrine-Diseases/Pregnancy_Thyroid_Disease_508.pdf
[5] National Institute Of Diabetes And Digestive And Kidney Diseases, Thyroid Disease & Pregnancy, https://www.niddk.nih.gov/health-information/endocrine-diseases/pregnancy-thyroid-disease
[6] American Thyroid Association, Iodine Deficiency,  https://www.thyroid.org/iodine-deficiency/
[7] Health, 4 Things You Should Do To Keep Your Thyroid Healthy, https://www.health.com/condition/thyroid/healthy-thyroid-0
[8] Healthline, Are Goitrogens In Food Harmful? https://www.healthline.com/nutrition/goitrogens-in-foods