Underactive Thyroid: What Are Normal TSH Levels?
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 TSH levels should be and how it relates to hypothyroidism.
What is TSH?
TSHTSH for Hashimotos disease and hypothyroidism 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 TSH 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).
Several other factors can also influence TSH levels:
-Inflammation of the thyroid gland
-Deficiency or excess of iodine in the diet
-Poisonous substances and radiation exposure
-Certain medications- antidepressants, cholesterol lowering drugs, chemotherapy drugs, steroids
-Summary: Current levels of thyroid hormone in the blood significantly dictates TSH levels, although there are other factors to influence it too.
Normal TSH levels
Normal TSH levels for the average adult range from 0.4-4.0 mIU/L (milli-international units per liter) (3).
However, many organisations agree that a reading of 2.5 or less is truly ideal, with anything 2.5 – 4.0 mIU/L considered at risks.
For those on thyroxine, goal TSH level is between 0.5 to 2.5 mU/L.
The reference ranges alter slightly as we grow older and if you are pregnant:
TSH levels for premature birth (28-36 weeks)
0.7 – 27 mIU/L
TSH levels 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
TSH levels for adults
21-54 years: 0.4-4.2 mIU/L
55-87 years: 0.5-‘8.9 mIU/L
TSH levels 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 TSH levels range from 0.4 – 4.0 mIU/L for the average adult.
High TSH Levels
A TSH reading above 4.0 mIU/L is considered high (elevated).
High TSH levels typically indicates an underactive thyroid gland, which produces too little thyroid hormone. This is known medically as hypothyroidism.
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
A TSH reading below 0.4 mIU/L is considered low.
Low TSH 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 Relying on TSH
TSH 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 TSH 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 TSH 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.
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:
-Thyroid Peroxidase Antibodies