TSH and So-Called “Normal” TSH Levels: A Non-Sciency Guide

What Is TSH and What Are Normal TSH Levels

[Last updated 5th November, 2018]

TSH is a hormone that controls thyroid gland activity.

It’s typically used as a marker of thyroid health, but what are normal TSH levels?

This article explores what your levels should be and how it relates to hypothyroidism.

Note that each section in this article has a ‘summary’ box (like this one) to save time. Below this box is a contents menu to help you navigate directly to a particular section.

What is TSH?

What is TSH?TSH is a hormone that controls thyroid function. It stands for Thyroid Stimulating Hormone.

It is actually produced by the brain’s pituitary gland, but stimulates production of the hormones triiodothyronine (T3) and thyroxine (T4) in the thyroid gland.

The amount and balance of these hormones affects almost every physiological process in the body, particular your body’s metabolism (1).

Summary: TSH is a pituitary hormone that stimulates or inhibits the production of thyroid hormones from the thyroid gland.

What Makes TSH Fluctuate?

The release of TSH is first stimulated by a hormone called TRH.

Once the thyroid stimulating hormone makes its way to your thyroid, levels are largely dictated by the amount of T3 and T4 in your blood.

When T3 and T4 levels are low, the body produces more TSH to stimulate the thyroid. But when T3 and T4 levels are high, the body produces less (2).

 

TSH cycle

Image source. Click to enlarge.

 

Several other factors can also influence TSH levels:

  • Inflammation of the thyroid gland
  • Genetics
  • Deficiency or excess of iodine in the diet
  • Poisonous substances and radiation exposure
  • Pregnancy
  • Certain medications- antidepressants, cholesterol lowering drugs, chemotherapy drugs, steroids
  • Thyroid cancer

Summary: Current levels of thyroid hormone in the blood significantly dictates your levels, although there are other factors that influence it too.

Hypothyroidism: What is TSH Level?

Due to the symbiotic nature they have with thyroid hormones, TSH levels in the blood can theoretically be a good marker of thyroid health.

This TSH levels chart presents a simplified version of what different readings can indicate.

TSH LevelsT3 and T4 LevelsDisease Condition
HighHighTumor of pituitary gland
LowLowSecondary hypothyroidism
LowHighGrave’s disease
HighLowHashimoto’s disease

Those with Graves’ disease (an overactive thyroid) have high low TSH and subsequent high thyroid hormone levels. Hashimoto’s disease (underactive thyroid) is the opposite.

Summary: Low TSH typically indicates excessive thyroid hormone levels, while high TSH indicates thyroid hormone deficiency.

Normal TSH levels

Normal TSH levelsWhat should your TSH level be?

Normal TSH levels for the average adult range from 0.4 ‑ 4.0 mIU/L (milli-international units per liter) (3).

However, many organizations agree that a reading of 2.5 or less is truly ideal for normal thyroid levels, with anything 2.5 – 4.0 mIU/L considered “at risk”.

For those on thyroxine, the ideal TSH normal range is between 0.5 to 2.5 mU/L.

The reference ranges alter slightly as we grow older and if you are pregnant:

TSH Normal Range By Age:

For premature birth (28‑36 weeks)

  • 0.7‑27 mIU/L

For children

  • Birth to 4 days: 1‑39 mIU/L
  • 2‑20 weeks: 1.7‑9.1 mIU/L
  • 21 weeks to 20 years: 0.7‑64 mIU/L

For adults

  • 21‑54 years: 0.4‑4.2 mIU/L
  • 55‑87 years: 0.5‑8.9 mIU/L

During pregnancy

  • First trimester: 0.3‑4.5 mIU/L
  • Second trimester: 0.3‑4.6 mIU/L
  • Third trimester: 0.8‑5.2 mIU/L

Small variations in results can occur depending on the laboratory and its methods used, as well as the time of day your blood was taken.

Summary: Normal levels range from 0.4 ‑ 4.0 mIU/L for the average adult.

High TSH Levels

A level reading above 4.0 mIU/L is considered high (elevated).

What is the reason for high TSH levels?

High levels typically indicates an underactive thyroid gland, which produces too little thyroid hormone. This is known medically as hypothyroidism.

High TSH levels

Common causes of hypothyroidism include an autoimmune disease (known as Hashimoto’s disease), radiation treatment, or surgical removal of the thyroid gland.

Replacing thyroid hormone and altering your diet are crucial for the safe and effective treatment of an underactive thyroid.

Summary: High TSH levels for the average adult are 4.2 mIU/L and over. This reading typically indicates an underactive thyroid.

Low TSH levels

normal TSH levelsA reading below 0.4 mIU/L is considered low.

What does it mean when your TSH is low?

Low levels typically indicates an overractive thyroid gland, which produces too much thyroid hormone. This is known medically as hyperthyroidism.

It can be caused by an autoimmune disease (known as Graves’ disease), goiter, excessive iodine in the body, or an overdose of synthetic thyroid hormone.

Initial hyperthyroidism treatment can involve anti‑thyroid medications and radioactive iodine to slow down thyroid hormone production. Most respond well to hyperthyroidism medications and are treated successfully.

Summary: Low TSH levels for the average adult are less than 0.2 mIU/L. This reading typically indicates an overractive thyroid.

The Problem With Solely Relying on TSH Tests

normal TSH levelsTSH is the most well-studied marker for judging thyroid health and function.

It has been the gold standard test for decades, and is considered the most sensitive and accurate indicator by most endocrinologists and other doctors.

However, more recent research indicates our systematic reliance on it is missing the mark. This leaves a lot of hypothyroid cases either misdiagnosed or undiagnosed.

Some clinical studies have found that both T3 and TSH levels can decline at the same time, particularly in obese individuals that lose weight (4, 5).

That means T3 levels can be low, yet TSH will remain in the normal range.

Certain medications, such as metformin, are also known to independently lower TSH levels in diabetics and PCOS patients with thyroid issues (6).

These variables are just the tip of the iceberg, but highlight why TSH is not completely reliable on its own. Considering the pituitary gland (which produces TSH) is unique in its function, it makes sense that some metabolic processes and outside stressors can influence TSH activity.

This is something to discuss with your doctor if your readings are high-normal, yet you still feel seriously unwell.

Summary: Several external stressors are known to influence TSH levels, independently of thyroid hormone levels. This means TSH on it’s own is not always a reliable indicator of thyroid health.

TSH Tests at Home

It’s not uncommon for doctors to skip over TSH testing if they do not believe there is a thyroid issue.

Fortunately, there is reliable way to measure thyroid stimulating hormones yourself with a home-testing kit. It’s FDA approved to be as accurate as a blood draw test, and results are sent direct to you by email or phone call.
TSH test
Click here to take a look on Amazon (aff link).

Have a read of the interesting reviews, as there’s the occasional comment that results are slow to get back.

Additional Tests For Thyroid Health

Given the potential inaccuracies with TSH on its own, comprehensive screening of thyroid health should ideally include these 6 tests:

  • TSH
  • Free T3
  • Free T4
  • Reverse T3
  • Thyroid Peroxidase Antibodies
  • Thyroglobulin Antibodies

Your doctor should be aware of these tests.

(Know that I cannot give personal medical advice. Please speak with your doctor.)

Wish you could go back to how you felt before thyroid symptoms?

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Introducing my Thyroid “Time Machine” meal plan. Click here to learn more.

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What Next?

Weight gain and low energy levels left you feeling frustrated? Check out my guide: How To Lose Weight With An Underactive Thyroid: Your 6-Step Guide

Looking for a free meal plan? See this article: 14-Day Meal Plan For Hypothyroidism And Weight Loss. Or simply download the full plan and shopping list here:

Further Reading

What is T4 and T3: The Complete Guide

TSH is a hormone that controls thyroid gland activity. This article explores what normal TSH levels are and how it relates to hypothyroidism.

About Joe Leech, Dietitian (MSc Nutrition & Dietetics)

Joe Leech is a university-qualified dietitian from Australia.

He graduated with a Bachelor's degree in exercise science, followed by a Master's degree in Nutrition and Dietetics in 2011.

Learn more about him on the About page