TSH level is often a misleading test for the assessment of thyroid function especially if you have hypothyroidism or hyperthyroidism. It is a useful test only in the follow-up of thyroid cancer patients.
What Is TSH?
TSH stands for Thyroid Stimulating Hormone. It a hormone produced by the specialized cells in the pituitary gland. It is not your thyroid hormone. In fact, it is an indirect way to assess the level of the thyroid hormones. It has a reciprocal relationship to the thyroid hormones: T4 ( Thyroxine) and T3 ( Triiodothyronine). In other words, your TSH gets low because your T4 and T3 are high and it goes high, if your T4 and T3 are low. While this reciprocal relationship may be true in experimental animals in the laboratory setting, this relationship often does not hold true in the clinical practice.
TSH is an indirect estimation of your thyroid hormone status. And may be affected by a number of reasons other than your T4 and T3, as I explain later in this article.
Physicians are typically trained to adjust the dose of your T4 (Levothyroxine which is also available with the brand names of Synthroid, Levoxyl—) based on your TSH level. In doing so, your physician will not pay any attention to the actual T4 and T3 level or your symptoms. In other words, your physician treats TSH, not you or your thyroid levels. Obviously, it is a flawed approach. Let me further elaborate on it.
Low TSH But Normal Free T3 and Free T4 in Hypothyroid Patients
Almost everyday I see a hypothyroid patient whose physician has been following the patient with TSH (alone or in combination with Free T3 and Free T4.) Often, these patients have low TSH but normal Free T4 and Free T3. When a physician sees this kind of thyroid blood test, he/she often disregards the normal Free T4 and Free T3 levels, which are the actual thyroid hormones. How ironic!
In stead, your your physician will typically lower the dose of Levothyroxine, which will raise TSH into normal range. Now your physician is pleased as your TSH is in the normal range. But you will start to feel worse with the symptoms of hypothyroidism such as fatigue, weight gain, foggy thinking, depressed mood, excessive hair loss….
Your physician will likely blame your symptoms of hypothyroidism on some other reason. He may even place you on some drug to control your symptoms. You may even be referred to other specialists. In this way, your physician is, in fact, worsening your hypothyroidism inadvertently. But he does not realize it because he blindly follows the drumbeat of TSH without ever looking at it scientifically.
Low TSH But Normal Free T3 and Free T4 in Hyperthyroid Patients
In case of hyperthyroidism, physician typically follow the drumbeat of TSH. For example, your physician may be treating your hyperthyroidism with medications such as methimazole. Your physician will typically monitor your TSH to see the effect of methimazole on your hyperthyroidism. As long as your TSH is low, your physician will be convinced that you are still having hyperthyroidism even though your symptoms of hyperthyroidism may be completely resolved. Some more conscientious physician may order a Free T4 (and rarely a Free T3) level. But even they will disregard them if these turn out to be normal. How ironic! Your physician disregards your clinical status as well as your normal thyroid level, but believes in an indirect thyroid test called TSH. Why? Because that’s what your physician has been trained to do.
Your physician will typically keep increasing the dose of methimazole until your TSH is in the normal range. By now, your Free T3 and Free T4 may be into the hypothyroid range. You feel terrible – fatigue, weight gain, foggy thinking. But your physician is pleased that your TSH is normal. And honestly believes that your symptoms may be due to some other medical disorder such as depression. He may even place you on an anti-depression medication. I have – in fact – seen a number of this kind of messed-up patients in my clinical practice.
Some physicians even recommend Radioactive Iodine to their patients who have low TSH, normal FT4 and normal FT3 and no symptoms of hyperthyroidism. These patients often end up losing their thyroid gland and become hypothyroid for the rest of their life.
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 inside the brain. Because it responds to your T3 level in the pituitary gland. But it does not necessarily indicates what is happening in the rest of the body. Therefore, your pituitary gland may be normal, but not the rest of the body. I have seen this phenomenon so many times in hypothyroid patients who decide to seek my advice, because their physician keep telling them “there is nothing wrong with your thyroid as your TSH is in the normal range.”
Iodine Deficiency Prevents a Rise in TSH level
There are a number of conditions which prevent the usual rise in TSH due to hypothyroidism. Deficiency of iodine is one such common cause. Iodine plays an important role in the normal functioning of a number of endocrine glands. These glands include thyroid, pituitary, testes, breasts 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, breasts 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 in my patients. I was surprised to find that 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 the normal upper limit of 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 their 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
Production of TSH takes place in the pituitary gland under the guidance of another hormone called TRH (Thyrotropin Releasing Hormone.) TRH is produced in the hypothalamus, an endocrine gland sitting on top of the pituitary gland. If something happens to your hypothalamus/pituitary, such as surgery or radiation to the pituitary and hypothalamic area in the brain, then your TSH goes down. Sometimes your pituitary may undergo necrosis due to infections or acute hemorrhage, which causes TSH to be low which 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) because he always interprets low TSH as a sign of hyperthyroidism. Consequently, he may inappropriately lower the dose of your thyroid pill and make you even more hypothyroid.
Drugs can Affect TSH level
Steroids such as Prednisone, Methylprednisone, Dexamethasone, can lower your TSH level, a basic medical fact. However, many physicians are often unaware of the TSH-reducing effect of the steroids. Consequently, a physician may interpret low TSH as an indicator of too much thyroid and lower your dose of thyroid medication. Here are a few examples:
- You may receive an epidural shot of steroids a few days before your thyroid blood test. Your TSH may be low simply because of the steroid shot. But your physician may mistakenly think your thyroid is high and may inadvertently lower your dose of the thyroid medicine, making you hypothyroid. And actually worsening your muscle aches and pain.
- You may receive a course of prednisone for your asthma exacerbation, which will lower your TSH level. Your physician may mistakenly think your thyroid is high and may inadvertently lower your dose of the thyroid pill, making you hypothyroid, which will make you more tired.
- You may receive dexamethasone while undergoing chemotherapy, which will lower TSH. Your physician may mistakenly think your thyroid is high and may inadvertently lower your dose of the thyroid medicine, making you hypothyroid, which will exacerbate your fatigue.
Metformin is perhaps the most commonly used drug to treat Type 2 diabetes. Many type 2 diabetics also have hypothyroidism. Recent studies have suggested that metformin can lower serum TSH level (1). Your physician may mistakenly think your thyroid is high and may inadvertently lower your dose of the thyroid medicine, making you hypothyroid, which will make you gain more weight and worsen your diabetes.
Certain seizure medications
Certain antiepileptic medications including carbemazepine, oxcarbemazepine and valproic acid may lower TSH level (2).
Sertraline(Zoloft) can increase TSH level while Reboxetine (Edronax) can significantly reduce it (3)
Dopamine infusion is often used in critically ill patients. It can lower TSH level. Consequently, your physician may mistakenly lower the dose of your thyroid hormone.
Bromocriptine is a drug that is used to treat pituitary tumors. It is also approved as a treatment for Type 2 diabetes. This anti-diabetic medicine has brand name of Cycloset. Bromociptine can lower your TSH level. Consequently, your physician may mistakenly lower the dose of your thyroid hormone.
Somatostatin analogs (brand name Octreotide, Sandostatin) suppress serum TSH . Physicians use these drugs to treat acromegaly, tumors secreting vasoactive intestinal peptide, carcinoid tumors, and glucagonomas.
Rexinoids are a subclass of vitamin A derivate drugs, or retinoids. Bexarotene (brand name Targretin) is the only rexinoid currently approved for clinical use, primarily for treatment of cutaneous T cell lymphoma. It can lower your TSH level. Consequently, your physician may mistakenly lower the dose of your thyroid hormone.
TSH is an indirect assessment of your thyroid hormone status. Many factors including commonly used drugs can affects it. Physicians typically do not realize these pitfalls in the interpretation of TSH. Then, they may inadvertently make wrong decisions simply because they did not realize that TSH may be a misleading test.
- Vigersky RA, Filmore-Nassar A, Glass AR. Thyrotropin Suppression by Metformin. J.Clin.Endocrinol.Metab. 2006;91(1):225–7
- Miller J, Carney P. Central Hypothyroidism With Oxcarbazepine Therapy. Pediatr.Neurol. 2006;34(3):242–4.
- Eker SS1, Akkaya C, Sarandol A, Cangur S, Sarandol E, Kirli S. Effects of various antidepressants on serum thyroid hormone levels in patients with major depressive disorder. Prog Neuropsychopharmacol Biol Psychiatry. 2008 May 15;32(4):955-61
For these reasons, I do not check TSH in my thyroid patients. The only exception is follow-up of the thyroid cancer patients, where we want to suppress TSH.
What is the best test for thyroid function? Free T3 and Free T4. For details, please refer to my book ” Hypothyroidism and Hashimoto’s Thyroiditis”