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. TSH is an indirect way to assess the level of the thyroid hormones. TSH supposedly 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 your TSH 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.
Sadly, your physician has been told to go by TSH as the absolute truth. We can call it the TSH drumbeat. Physicians are 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 without any regard to the actual T4 and T3 level in your blood or your symptoms. This is a flawed approach as explained in the following section.
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, they have low TSH but normal Free T4 and Free T3. When a physician sees this kind of blood test, he/she often disregards the normal Free T4 and Free T3 levels, which are the actual thyroid hormones. How ironical! In stead, your physician will go with the TSH drumbeat, which 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 explained later in this article. Your physician will lower the dose of Levothyroxine, which will raise TSH into normal range. And you will start to feel worse with the symptoms of hypothyroidism such as fatigue, weight gain, foggy thinking, depressed mood, excessive hair loss….
Now your physician is pleased because he/she has done what he/she was trained to do. Your physician, in fact, believes that your symptoms of hypothyroidism such as fatigue, weight gain, depressed mood, excessive hair loss…. are not due to hypothyroidism but some other reason, and may even place you on some drugs to control these symptoms, which will have their own side-effects. You may even be referred to other specialists. In this way, your physician is, in fact, causing the symptoms of hypothyroidism inadvertently as he/she is blindly following the drumbeat of TSH as the gold standard to monitor your thyroid hormone status.
Low TSH But Normal Free T3 and Free T4 in Hyperthyroid Patients
In case of hyperthyroidism as well, physician typically follow the drumbeat of TSH as the gold standard. For example, your physician is 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 these levels if these turn out to be normal. How ironic! Your physician disregards your clinical status as well as your normal thyroid level, as he/she is stuck on an indirect thyroid test called TSH. Why? Because that’s what your physician has been told to do. I am so amazed at the power of this brainwash.
Your physician will usually increase the dose of methimazole which will eventually raise TSH into the normal range, but at the expense of lowering your Free T3 and Free T4 into the hypothyroid range. You feel terrible – fatigue, weight gain, foggy thinking. But your physician is pleased that your TSH is normal. I have actually seen a number of these messed-up patients in my clinical practice.
I have seen even patients who have been advised to receive Radioactive Iodine who have low TSH, normal FT4 and normal FT3 and have no symptoms of hyperthyroidism.
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 gland may be normal, but not the rest of the body, as far as the thyroid hormones are concerned. 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.” For more details, please refer to my book, ” Hypothyroidism and Hashimoto’s thyroiditis.
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 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/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
Production of TSH takes place in the pituitary gland under the guidance of another hormone called TRH (Thyrotropin Releaing 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. 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) because he/she always interprets low TSH as a sign of hyperthyroidism. Consequently, he/she may inappropriately lower the dose of your thyroid pill.
Drugs can Affect TSH level
Steroids can lower your 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 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 medicine, 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 can lower your TSH levels
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 antiepileptic medications can lower your TSH level
Certain antiepileptic medications including carbemazepine, oxcarbemazepine and valproic acid may lower TSH level (2).
Antidepressants can affect your TSH level
Sertraline(Zoloft) can increase TSH level while Reboxetine (Edronax) can significantly reduce TSH level (3)
Dopamine can lower your TSH level
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.
Bromociptine can lower your TSH level
Bromocriptine is a drug that is used to treat pituitary tumors. Recently, 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 suppress your TSH level
Somatostatin analogs (brand name Octreotide, Sandostatin) suppress serum TSH . Somatostatin analogs are used for treatment of acromegaly, tumors secreting vasoactive intestinal peptide, carcinoid tumors, and glucagonomas.
Rexinoids suppress your TSH level
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
- 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”