Thyroid Lab Value RelationshipsThe down and dirty of TSH, T3 and T4 lab values.

A lot of people ask want to know how to understand the levels for Thyroid lab values. More specifically the relationships. I think these questions are confusing for one specific reason. The lab values aren’t intuitive.

Let me explain.

Thyroid Stimulating Hormone

High values

There are a few causes for High TSH.

  1. Hypothyroidism. This is the tricky part. It seems like if you have too much Thyroid stimulating hormone you would have hyperthyroidism. Where actually the reverse is the case. So the first thing to remember if TSH goes up thyroid function goes down. It is an inverse relationship. Hashimoto’s thyroiditis is the most common cause of primary hypothyroidism.
  2. Another cause of high TSH is a pituitary gland tumor that is making too much TSH though this is uncommon.
  3. The third most likely cause is that the patient is not taking enough thyroid hormone medicine to treat their underactive thyroid gland.

Low values

There are a few causes for Low TSH.

  • Hyperthyroidism. Remember if TSH is low the Thyroid will be overactive. There are a few causes of hyperthyroidism: Graves disease, toxic multinodular goiter or a even a benign tumor (toxic nodule).
  • Secondary Hyperthyroidism, this is where there is damage to the Pituitary gland that causes it to not make TSH.
  • Maybe the patient is taking too much thyroid hormone medicine to treat their underactive thyroid gland.
  • High TSH can also be caused by first trimester pregnancy.

T3 and T4 Lab values

A low T4 value could mean a diseased thyroid or a pituitary gland that is not functioning. If the TSH is high then that means the pituitary gland is functioning (it is releasing TSH) therefore the thyroid gland is diseased and responsible for the low T4 value.

So if you have a low T4 level and a low TSH level the the most likely cause is the pituitary gland.

So for all our intents and purposes we are concerned with the Thyroid itself. So if there is high TSH and low T4 then the thyroid is the culprit and you will see this as hypothyroid.

T3 will generally correlate with T4 levels.

So what do we take away from this. Here is what to remember.

If TSH goes up you will have thyroid function that goes down. If thyroid function goes down then you have lab values for T3 and T4 that will go down.


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I am the founder and lead Instructor here at I strive to help those that are entering the field of Ultrasound to be the best that they can be. Ultrasound is my passion.

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