TSH stands for Thyroid Stimulating Hormone. It’s the signal that stimulates the thyroid gland to produce:

• T4 (Thyroxine) – Main circulating thyroid hormone
• T3 (Triiodothyronine) – the active form of thyroid hormone your body uses (mainly converted from T4 in tissues.)

The pituitary gland secretes TSH when thyroid hormone levels drop. Think of TSH as the signal the pituitary gland sends to the thyroid to produce more hormones. When your thyroid hormones are high, the pituitary gland secretes less TSH to signal the thyroid to slow down thyroid hormone production.

TSH Levels

TSH levels indicate and help doctors classify thyroid problems like Hypothyroidism (slow thyroid), Hyperthyroidism (fast thyroid), Hashimoto’s or Graves’ disease.

Here’s what different TSH levels mean:

High TSH – Your thyroid is underactive, working slower than normal. Think of it like a thermostat where temperature drops and the body is cranking up the dial (TSH) hoping the thyroid works harder. If you were to turn up the thermostat in your home, but it’s still cold, then you can start to determine that there is a problem with the furnace or the thermostat. That’s how you begin to find out if you are dealing with primary or secondary hypothyroidism.

Low TSH – Your thyroid is overactive, working faster than normal. Think of it like the body is getting too hot and the thermostat dial is being turned down to get the thyroid to slow down. If you were to turn down your thermostat at home, but it’s still too hot, then you begin to wonder if the A/C is out or if the thermostat is broken.

Normal TSH – In theory, if you set your thermostat at a temperature at home and the temperature matches that, then everything is in working order. However, there are cases where your TSH may show up as normal in blood test, but you don’t feel normal. This could be cellular hypothyroidism. Conventional doctors don’t recognize cellular hypothyroidism, but some naturopaths and functional medicine practitioners do.

What is TSH in Blood Test

Free T4 gives Thyroid Stimulating Hormone (TSH) levels context to get a meaningful interpretation from lab tests. Doctors recognize four scenarios when it comes to TSH levels:

Central Hypothyroidism – Low TSH + Low/Normal T4

The pituitary gland responds to low T4 by increasing TSH. If TSH levels remain low when T4 is low or normal, then it could mean there is a breakdown in communication between circulating T4 and the pituitary gland. The pituitary gland fails to secret TSH as expected when T4 levels are low.

Primary Hyperthyroidism – Low TSH + High T4

Overproduction of thyroid hormone T4 can suppress TSH levels. High T4 should lower TSH to bring into balance, but TSH should not remaim low. Low TSH should reduce T4 production and come back to balance. When TSH remains low and T4 remains high, it indicates the thyroid is over working.

Subclinical Hyperthyroidism – Suppressed TSH + Normal T4

Normal T4 should be accompanied by normal TSH. Suppressed TSH (trending lower) informs doctors of an early sign of hyperthyroidism.

Subclinical Hypothyroidism – Elevated TSH + Normal T4

Normal T4 levels means TSH should also be normal. The body increases TSH levels to compensate if the thyroid gland isn’t producing normal T4 levels. It means the thyroid gland needs extra stimulation from the pituitary gland to maintain normal thyroid levels.

This is where people often hear, “your levels are bad, but not bad enough to be treated”

Primary Hypothyroidism – High TSH + Low T4

Once TSH levels get high enough, it means that the pituitary gland having to work extra hard to get the thyroid gland to maintain normal thyroid hormone production.

Thyroid Hormone Resistance – High TSH + High T4

If the body is producing a lot of T4, the expected result would be to have low TSH to bring things back into balance. When you have both high TSH levels and high T4 levels, then the pituitary gland is not getting the signal from T4 to lower secretion of TSH.

What is TSH ultrasensitive

TSH ultrasensitive is the standard of care in modern thyroid evaluation capable of measuring lower TSH levels below 1. It can detect TSH levels as low as 0.01 mIU/L or even lower.

What is TSH w/reflex to ft4 (TSH reflex free T4)

Reflex testing is a lab testing strategy that only tests for Free T4 if TSH levels are abnormal. If TSH levels are normal, then it presumes that thyroid function is normal.

TSH test normal range for female

Normal TSH ranges can vary depending on where you get your labs from due to variations in testing techniques, equipment, etc.

The general reference range is 0.4 to 4.0 mIU/L.

0.5 to 2.5 is the Optimal or Functional range that tells endocrinologists, integrative or functional medicine practitioners things are normal.

Pregnant women expect lower TSH levels due to hormonal changes that can vary depending on the trimester:

First Trimester – 0.1 to 2.5

Second Trimester – 0.2 to 3.0

Third Trimester – 0.3 to 3.5

The American Thyroid Association supports these ranges.

What is considered a dangerously high TSH level

TSH levels alone don’t tell you the full picture. You must look at TSH levels in conjunction with Free T4, Free T3, thyroglobulin antibodies, symptoms, age, etc., to get a full picture.

Doctors categorize Dangerously high TSH in two groupings:

Overt Hypothyroidism – 10 to 20 mIU/L

Severe or prolonged hypothyroidism – 20 to 40+ mIU/L