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.
The thyroid gland produces thyroid hormone. When it functions properly, the thyroid is part of a feedback loop with your pituitary gland. First, the pituitary senses the level of thyroid hormone that the thyroid has released into the bloodstream. The 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 the thyroid, for whatever reason -- illness, stress, surgery, obstruction, for example -- does not produce enough thyroid hormone, the pituitary detects this reduction in thyroid hormone, and it moves into action. The pituitary then makes MORE TSH, to help trigger the thyroid to produce more thyroid hormone. This is the pituitary's effort to return the system to "normal" and normalize thyroid function.
There, a TSH that is higher than normal suggests a thyroid that is underactive and not doing its job of producing thyroid hormone. So, in general, HIGHER TSH = UNDERACTIVE THYROID / HYPOTHYROIDISM.
If the thyroid is overactive and producing too much thyroid hormone -- due to disease, or taking too high a dose of thyroid hormone replacement drugs -- the pituitary senses that there is too much thyroid hormone circulating. The pituitary then usually slows or shuts down TSH production, so that the thyroid will slow down its production of 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 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. As of 2008, at most labs in the U.S., the normal reference range is approximately 0.5 to 5.0. So levels below 0.5 are considered possible evidence of hyperthyroidism, and levels above 5.0 would be considered possible evidence of hypothyroidism. There is, however, an ongoing, five-year dispute among thyroid experts, because as of late 2002, some endocrinologists believed that the range should be narrowed significantly, to 0.3 to 3.0. Endocrinologists have not yet reached agreement about a standardized normal reference range for the TSH thyroid test, despite the fact that they consider this test the gold standard test to use in diagnosing and treating many thyroid problems.
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 potentialy "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.)
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.
If you're being treated with thyroid hormone drugs (such as Synthroid, Levoxyl, Armour, Thyrolar, etc.), and your TSH comes back as too low, your doctor may suggest lowering your dose, or running other tests such as Free T4 and Free T3.
When TSH tests come back as higher, that suggests HYPOthyroidism (underactive), and doctors may suggest an INCREASE in your medication dosage.