Low TSH on Thyroid Blood Test

Thyroid Blood Test--What Should Your Numbers Be?

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Do you think to yourself, "What should my thyroid levels be?"  If you do, you are not alone.

Readers regularly write to me asking that I explain "why, when my TSH results show that my TSH is low, does the doctor want to LOWER instead of RAISE my medicine?"

Or, "doesn't a high TSH mean I have too much thyroid? And if that's the case, why is the doctor INCREASING my thyroid hormone medicine?"

This is one of those issues about thyroid treatment that tends to confuse people.

So here's a basic explanation.

Your thyroid gland produces thyroid hormone. When it functions properly, your thyroid is part of a feedback loop with your pituitary gland that involves several key steps.

1. First, your pituitary senses the level of thyroid hormone that the thyroid has released into the bloodstream.

2. Your pituitary then releases a special messenger hormone, known as "Thyroid Stimulating Hormone" (abbreviated as TSH). The role of TSH is to stimulate the thyroid to release more thyroid hormone.

When your thyroid, for whatever reason -- illness, stress, surgery, obstruction, for example -- doesn't or can't produce enough thyroid hormone, your pituitary detects this reduction in thyroid hormone, and it moves into action by making MORE TSH, which then triggers your thyroid to make more thyroid hormone. This is the pituitary's effort to return the system to "normal" and normalize thyroid function.



So, a TSH that is higher than normal suggests a thyroid that is underactive and not doing its job of producing enough thyroid hormone. To generalize, then, HIGHER TSH = UNDERACTIVE THYROID / HYPOTHYROIDISM.

3. If your thyroid is overactive and producing too much thyroid hormone -- due to disease, or taking too high a dose of thyroid hormone replacement drugs -- your pituitary senses that there is too much thyroid hormone circulating, and slows or shuts down TSH production, so that the thyroid will slow down its production of the hormone.

This drop in TSH is an attempt to return circulating thyroid hormone levels to normal.

So, a test to measure the amount of TSH in your system will usually show lower than normal TSH when the thyroid is overactive.

So, in general, LOWER TSH = OVERACTIVE THYROID / HYPERTHYROIDISM.

During diagnosis, most doctors use the TSH test to evaluate your thyroid function and determine the optimal course of treatment. [NOTE: however, that some practitioners feel that relying solely on TSH -- a pituitary hormone -- without also evaluating the circulating levels of actual thyroid hormones T4 and T3 -- may not be able to detect more subtle thyroid problems, or conditions that are resulting from the improper conversion of thyroid hormones. TSH is also not necessarily sufficient to monitor hypothyroidism during pregnancy. For these reasons, some practitioners also include other valuable blood tests, including T4, T3, Free T4, Free T3, Reverse T3, and antibodies tests.]

A major hitch in this connection of TSH to hypothyroidism and hyperthyroidism is an ongoing disagreement in the medical world over the reference range for the TSH test.

Levels below 0.5 are considered possible evidence of hyperthyroidism, and levels above 5.0 are typically considered possible evidence of hypothyroidism, but some experts feel this range is too broad, and that it should be narrowed significantly, to 0.3 to 3.0. This is an issue that is still under debate. 

When you are being treated for hypothyroidism with thyroid hormone replacement drugs, doctors will typically attempt to medicate you into this so-called "normal" reference range -- of a TSH from .3/.5 on the low-end, to 3.0/5.0 on the high-end. (Patients who have had thyroid cancer, however, are often given suppressive doses that maintain TSH near to 0 in order to prevent cancer recurrence).

So, when you've gone for a checkup, and your TSH comes in below normal (and your doctor does not have you on a suppressive dose of thyroid hormone), they may want to REDUCE your dosage of thyroid hormone, because levels below normal are considered potentially "hyperthyroid" (overactive.)

[Another point of controversy: Once on medication, some patients may find relief from hypothyroidism symptoms only when the TSH level drops below the normal range. In that case, some doctors will also check Free T4 and Free T3 levels, believing that it's acceptable for a patient to have a below-normal TSH, as long as Free T4 and Free T3 levels are normal.]

And if your TSH test comes in above normal, some doctors will want to INCREASE your dosage of thyroid hormone, because levels above normal are considered potentially "hypothyroid" (underactive.)

To recap:

LOW TSH suggests you are closer to HYPERthyroidism (overactive), and have too much thyroid hormone circulating.

HIGH TSH suggests you are closer to HYPOthyroidism (underactive), and you don't have enough thyroid hormone circulating.

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