The happy homeotherm

If you were a "cold blooded" poikilotherm unable to regulate internal body temperature, you would have to sun yourself on rocks to raise your body temperature, just like turtles and snakes. When it got cold, your metabolic rate would slow and you might burrow into the mud to hide.

You and I, however, are homeotherms, terrestrial animals able to regulate our own internal body temperature. Principal responsibility for keeping your body temperature regulated falls with the thyroid gland, your very own thermoregulatory "thermostat."

But internal body temperature, even in a homeotherm, varies with circadian rhythm: Highest temperature occurs in the early evening around 8 p.m.; the low temperature nadir occurs at around 4 a.m.

The notion that normal human temperature is 98.6 degrees Fahrenheit is a widely-held fiction, a legacy of the extraordinary experience of 19th century German physician, Carl Reinhold August Wunderlich, who claims to have measured temperatures of one million people using his crude, uncalibrated thermometer to obtain axillary (armpit) body temperatures.

Dr. Broda Barnes was a 20th century American proponent of using the nadir body temperature to gauge thyroid function. Like Wunderlich, Barnes also used axillary temperatures.

Modern temperature assessments have employed radiotransmitting thermistors that are swallowed, with temperatures tracked as the thermistor travels through the stomach, duodenum, small intestine, large intestine, rectum, then peek-a-boos back out. Such internal "core temperature" assessments have shown that:

--Axillary temperatures do not track with internal core temperatures very well, often veering off course due to external factors.
--Axillary temperatures are subject to ambient temperatures, such as room temperature, and are affected by clothing.
--Axillary temperatures are more susceptible to physical activity, e.g., increased with exercise or physical work.

Even right vs. left axillary temperatures have been shown to vary up to 2 degrees Fahrenheit.

Studies such as this demonstrate that normal oral temperature upon arising is around 97.2-97.3 degrees Fahrenheit. While we lack data correlating thyroid function with circadian temperature variation, the a.m. nadir does indeed, as Dr. Barnes originally suggested, seem to track thyroid status quite well: lower with hypothyroidism, higher with normal or hyperthyroidism.

I have been using 97.3 degrees F orally as the cutoff for confirming or uncovering thyroid dysfunction, particularly when symptoms or blood tests (TSH, free T3, free T4) are equivocal, a value that has held up well in the majority of cases. I find it helpful when, for instance, someone complains of cold hands and feet and has normal TSH (1.5 mIU/L or less in my view) but low free T3. An a.m. oral temperature of, say, 95.7 degrees F, suggests that there will be a favorable response to T3 supplementation. And it nearly always plays out that way.

Wouldn't it be interesting to know if there was insight into thyroid status provided by also examining the circadian behavior of temperature (e.g., height or timing of the peak)?

评论 (14) -

  • Anonymous

    2010/11/24 21:08:56 |

    perhaps a disturbed circadian rhythm results in excess prolactin and that to hypothyroidism... thanks to our modern lighting keeping us up till late at night.

  • Anand Srivastava

    2010/11/24 21:38:59 |

    The temperature over the day actually tracks the cortisol level. The T3 is present in the blood, but is taken up by the cells only when cortisol is present. In the morning till 12:00 the cortisol is highest. Before sleeping it dips to its lowest waking value. It will dip further in the night. 2 hours before sleeping it will start to rise. I guess the temperature tracks the cortisol with a delay of a couple of hours. Providing the lowest temperature immediately at waking up.

    There is a good temperature graph method by Dr.Rind to monitor thyroid function and also Adrenal function.

    It seems that thyroid is responsible for defining the body temperature, while adrenal is responsible for varying the temperature as per requirement.

    The test requires taking the temperature 3 times a day using an analogue meter (like the mercury thermometer). The temperature is taken at 3, 6, and 9 hours after waking, and then averaged. Remember to keep the thermometer in the mouth for around 10 minutes and don't do anything. Less will not be sufficient, and movement changes the temperature.

    This test is done for at least 5 days. The average temp will define the thyroid function. If it is too low, it indicates thyroid problem. If the variation day to day is too much then it indicates a cortisol problem.

    I am not sure what the average temperature should be but I think it should be at least 98.2F. The temperature variation has been recommended to be below .3F.
    Mine varies 1F. I have a lot of adrenal insufficiency symptoms.

  • LeonRover

    2010/11/24 21:47:14 |

    I am constantly astonished that population studies of common markers such as temperature and blood pressure have such low samples as in the hundreds.

    My mother would take my temperature when I was young and might call our GP if it was over 100 F.

    I had assumed (until recently) that decent statistically large databases, covering 000's of people, divided by gender, age, time of day, diseases etc., from which one might get some interesting were available.

    Alas, there is only Broda Barnes, or studies with 190 in sample.

    It is disturbing.

  • Daniel A. Clinton, RN, BSN

    2010/11/24 23:03:09 |

    Dr. Davis,
    I'm a big fan. It's refreshing for me to find people with a deeper understanding than me. Would you treat the hypothetical patient in your post with the normal TSH but low T3 with iodine supplementation before prescribing T3, or jump right to med therapy?
    It boggles my mind that a good chunk of hypothyroidism is simple iodine deficiency, but that truth is hidden from all but those who vigilantly try to seek it out.

  • Lori Miller

    2010/11/25 0:55:41 |

    Something that affects temperature in women: the time of the month.

  • Anonymous

    2010/11/25 2:58:00 |

    alot of low t3 is just leptin resistance.  Fix the leptin resistance with no sugars and starches, plenty of green veges, fish and a bit of lean meat, and the thyroid starts converting t4 back into t3 and not reverse t3.  I actually believe the t3 supplementation is like giving insulin to a T2DM- it just kills them quicker!

  • Anonymous

    2010/11/25 7:13:17 |

    How do you suggest to measure the oral basal body temperature?  30 years ago the suggestion was to use the axillary temp.  Now with the ear digital thermometers that would seem to be easier and more accurate.   We used 97.2 axillary, you suggest 97.3 orally, no problem but how do you suggest to measure

  • qualia

    2010/11/25 12:10:07 |

    totally agree with @anonymous - supplementing T3 is an extremely crude and potentially harmful (long term) measure. leptin and physical activity plays a huge role in the conversion of T4 to T3. also iodine, selenium and zinc status. listen to the last podcast by byron richards covering all of these correlations in his "thyroid health class" episode (mp3):

  • Dr. William Davis

    2010/11/25 14:20:43 |

    Thanks, Daniel. It is always worth trying iodine first; crudely estimated, about 30% of people will respond just to iodine. If that doesn't work, then a T3 preparation or a combination T4/T3 preparation is worth considering.

    Hi, Lori--Yes, indeed. A big effect. I advocate measuring it during the first 7 days after menstrual bleeding starts, the time when temperature is lowest.

    Anonymous--I actually have plenty of slender people (e.g., BMI <23), therefore presumably with normal leptin levels, who still display the low-T3 effect. I believe there is more to this issue.

  • steve

    2010/11/25 16:46:31 |

    what actually constitutes a low T3?
    Is it below the range the lab considers normal, with in the range, but at bottom 10% or the 40 to 50 % level of the range?

  • Anonymous

    2010/11/25 20:01:57 |

    Hi Dr Davis
    I have read several articles and studies that link lower T3 levels to calori restriction diets and to slower aging.
    Wouldn't T3 supplementation accelerate aging process?

  • Anonymous

    2010/11/26 20:17:45 |

    Hi Dr Davis
    sigh!  You are right on most things including about butter being insulinogenic (but I still eat it anyway) but you have not grasped the basic idea yet.

    BMI is such a crude tool.  It does not account for how much is fat and how much is muscle.  Very skinny people are highly leptin resistant (think the osteoporotic grandmother who also has IHD - most from the generation above us are skinny).  That is why BMI in critical care units is inversely related to mortality.  Lean and very muscular people are unlikely to be leptin resistant, though they may have a BMI of 26.  People with low BMI, or marathon runners with stress fractures are more likely to be leptin resistant.  People with BMIs of over 26 are usually  quite leptin resistant though there may be some exceptions to that rule.  We are all leptin/insulin resistant because this is how we age, but most of us are ageing too quickly.  
    Byron Richards, despite being a dreaded naturopath (and recommending wheat!), understands this connection very well:

    PS  my child is now off her t4! - we followed a modified rosedale.  Thyroid supplementation, testosterone supplementation, growth hormone treatment and insulin for  T2 do not treat the underlying cause, and will accelerate the ageing process and cause sarcopenia, whether you are fat or skinny.

  • Anonymous

    2011/2/16 17:23:35 |

    I have found this website helpful regarding thyroid

  • Paul

    2011/5/10 20:55:16 |

    I am hypothyroid and on T4 T3 combination therapy.  In 2010, I followed Michael Pollan's "Food Rules" less the whole grains: I was weight stable.  However, starting this year (and still following the organic, whole food approach) I also embarked on strict carb restriction.  I lost about 16 pounds and then I hit a plateau.  I also got a tooth abscess and malaise.  My annual thyroid tests showed a TSH of <1, but a T4 at the bottom of the range and a hypothyroid T3.  My T4 T3 have been revised upwards and I am staying with carbohydrate restriction.

    Perhaps this suggests that hypothyroid patients need to be cautioned when embarking on carbohydrate restriction. Or, perhaps I am a one-off.  Also, are (under-treated) hypothyroid people susceptible to carb diets as such diets may effectively reduce the extent of their thyroid deficiency?

CT scans and radiation exposure

CT scans and radiation exposure

The NY Times ran an article called

With Rise in Radiation Exposure, Experts Urge Caution on Tests at

“This is an absolutely sentinel event, a wake-up call,” said Dr. Fred A. Mettler Jr., principal investigator for the study, by the National Council on Radiation Protection. “Medical exposure now dwarfs that of all other sources.”

Where do CT heart scans fall?

Let's first take a look at exposure measured for different sorts of tests:

Typical effective radiation dose values

Computed tomography Milliseverts (mSv)

Head CT 1 – 2 mSv
Pelvis CT 3 – 4 mSv
Chest CT 5 – 7 mSv
Abdomen CT 5 – 7 mSv
Abdomen/pelvis CT 8 – 11 mSv
Coronary CT angiography 5 – 12 mSv

Non-CT Milliseverts (mSv)

Hand radiograph Less than 0.1 mSv
Chest radiograph Less than 0.1 mSv
Mammogram 0.3 – 0.6 mSv
Barium enema exam 3 – 6 mSv
Coronary angiogram 5 – 10 mSv
Sestamibi myocardial perfusion (per injection) 6 – 9 mSv
Thallium myocardial perfusion (per injection) 26 – 35 mSv

Source: Cynthia H. McCullough, Ph.D., Mayo Clinic, Rochester, MN

If you have a heart scan on an EBT device, then your exposure is 0.5-0.6 mSv, roughly the same as a mammogram or several standard chest x-rays.

A heart scan on a 16- or 64-slice multidetector device, your exposure is around 1.0-2.0 mSv, about the same as 2-3 mammograms, though dose can vary with this technology depending on how it is performed (gated to the EKG, device settings, etc.)

CT coronary angiography presents a different story. This is where radiation really escalates and puts the radiation exposure issue in the spotlight. As Dr. Cynthia McCullough's chart shows above, the radiation exposure with CT coronary angiograms is 5-12 mSv, the equivalent of 100 chest x-rays or 20 mammograms. Now that's a problem.

The exposure is about the same for a pelvic or abdominal CT. The problem is that some centers are using CT coronary angiograms as screening procedures and even advocating their use annually. This is where the alarm needs to be sounded. These tests, as wonderful as the information and image quality can be, are not screening tests. Just like a pelvic CT, they are diagnostic tests done for legimate medical questions. They are not screening tests to be applied broadly and used year after year.

Always be mindful of your radiation exposure, as the NY Times article rightly advises. However, don't be so frightened that you are kept from obtaining truly useful information from, for instance, a CT heart scan (not angiography) at a modest radiation cost.

Detail on radiation exposure with CT coronary angiograms on multidetector devices can be found at Hausleiter J, Meyer T, Hadamitzyky M et al. Radiation Dose Estimates From Cardiac Multislice Computed Tomography in Daily Practice: Impact of Different Scanning Protocols on Effective Dose Estimates. Circulation 2006;113:1305-1310, one of several studies on this issue.

Comments (8) -

  • Anonymous

    6/20/2007 1:13:00 AM |

    I had a calcium score scan on a 64-slice machine at the Morristown Hospital in New Jersey. No contrast was injected. The technician did three separate scans that included the lung, even thought I didn't for a lung scan. I wonder why three scans were taken. Does it mean that I had three times the radiation?

  • Dr. Davis

    6/20/2007 1:22:00 AM |

    Of course I can't comment specifically on what was done, but it is common practice to perform 1) a "scout" film for the technologist to identify the location of important "landmarks" like the sternum and the top and bottom of the heart to minimize the window of exposure, and 2) lung imaging as a routine part of  heart imaging, not necessarily an additional scan.

    If an additional and unrequested lung scan was performed, you may want to call and ask why this policy is in operation.

  • Anonymous

    6/21/2007 4:35:00 AM |

    What do you feel about yearly nuclear stress tests for people with CAD?  The radiation exposure seems high and the ability of a stress test to pick subtle changes in flow is low.  In the absence of symptoms it would appear that the common practice of nuclear stress tests for people with CAD is a questionable practice.

  • Dr. Davis

    6/21/2007 12:14:00 PM |

    I agree. The radiation is excessive. I tend to follow that route only when nothing else is possible. An alternative for stress testing is stress echocardiogram in its various forms, none of which involve radiation. They still suffer the other pitfalls of stress testing, of course, but do not involve radiation.

  • Mike

    12/20/2008 11:40:00 AM |

    I just launched a webiste that may answer some of your questions. It allows you to calculate your cancer risk based on studies you have had and answers some faq on radiation exposure and cancer.

  • Anonymous

    12/6/2009 12:52:26 AM |

    There are several ways to estimate your cancer risk - the best site for background information is probably the Image Gently campaign.

    The American College of Radiology has similar information pages for patients and the general public.

    To track your exposure, as Mike said there's the xrayrisk website.
    There's also a program for the iphone called Radiation Passport that tracks all of your radiation exposure and gives you the associated risk of developing cancer from your radiation exposure.

  • buy jeans

    11/3/2010 6:33:12 PM |

    CT coronary angiography presents a different story. This is where radiation really escalates and puts the radiation exposure issue in the spotlight. As Dr. Cynthia McCullough's chart shows above, the radiation exposure with CT coronary angiograms is 5-12 mSv, the equivalent of 100 chest x-rays or 20 mammograms. Now that's a problem.

  • Medical CT

    11/29/2010 4:34:03 AM |

    The CT scanner was originally designed to take pictures of the brain. Now it is much more advanced and is used for taking pictures of virtually any part of the body.

    The scanner is particularly good at testing for bleeding in the brain, for aneurysms (when the wall of an artery swells up), brain tumours and brain damage. It can also find tumours and abscesses throughout the body and is used to assess types of lung disease.