TSH – a misleading test for thyroid function
TSH stands for Thyroid Stimulating Hormone. It is not your thyroid hormone. TSH is an indirect way to assess the level of the thyroid hormone. Often, it is a misleading test for the thyroid function.
Normal TSH does not always mean normal thyroid function
TSH is a hormone produced by the pituitary gland in response to T3 level inside the pituitary gland. There is an inverse relationship between TSH and T3 level inside the pituitary gland. Consequently, if your pituitary gland senses T3 to be low, it produces more TSH, which is a message for the thyroid gland to produce more thyroid hormone. Conversely, if T3 is high, then pituitary gland produces less TSH, telling thyroid gland to produce less thyroid hormone. In this way, pituitary gland tries to regulate thyroid hormone production by the thyroid gland.
Your TSH may be in the normal range while you are actually hypothyroid
Your TSH is a reflection of what is going on in your pituitary gland, not necessarily what is happening in the rest of the body, such as your muscles, skin, fat, liver, kidneys and heart. Therefore, your pituitary may be normal, but not the rest of the body, as far as the thyroid hormones are concerned. I have seen it repeatedly in my patients.
Iodine Deficiency Prevents a Rise in TSH level
There are a number of conditions which prevent the usual rise in TSH due to hypothyroidism. Iodine deficiency is one such common cause. Iodine plays an important role in the normal functioning of a number of endocrine glands. These endocrine glands include thyroid, pituitary, testes and ovaries. When you are low in iodine, as most people are, your thyroid preferentially sucks up most of the available iodine. Consequently, there is little iodine left for the rest of the body, including pituitary, testes and ovaries. Less iodine leads to less than optimal functioning of the pituitary gland. Hence, TSH level does not rise despite relatively low T3 level inside the pituitary gland.
Years ago, I started to check for iodine deficiency/insufficiency. I found most of my patients were low in iodine despite eating a healthy diet. When I placed them on iodine supplement, their TSH went up, higher than 4.0 mIU/L. At the same time their Free T3 (FT3) and Free T4 (FT4) also went up, but within the normal range. It meant that the thyroid gland started to produce more thyroid hormone – Ft3 and Ft4, under the direction of higher level of TSH. If I were to just look at the TSH level, as most physicians do, I would erroneously diagnose my patients to be having worsening of hypothyroidism. In fact, they were no longer hypothyroid as demonstrated by a rise in their FT3 and Ft4 – the actual thyroid hormones.
TSH is low if your pituitary gland is unable to produce it
This happens in people who undergo surgery or radiation to the pituitary and hypothalamic area in the brain. Sometimes a large pituitary tumor or large doses of steroids may also cause TSH to be low. Sometimes pituitary may undergo necrosis due to infections or acute hemorrhage, which causes TSH to be low. Low TSH then leads to low thyroid hormone production by the thyroid gland-Hypothyroidism. But, your physician may mistakenly interpret it as a sign of too much thyroid hormone (hyperthyroidism), and may inappropriately lower the dose of your thyroid pill.
What is the best test for thyroid function? Refer to ” Hypothyroidism and Hashimoto’s Thyroiditis”