Thermoregulation and the Track Your Plaque Program

The regulation of body temperature - thermoregulation - is a reflection of the body’s capacity to adapt to a rapidly-changing external environment and maintain temperature within a narrow range. Deviations from this range can signal disruption of internal control.

Can this phenomenon provide insight into metabolic - specifically thyroid - health?
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If it weren’t for thermoregulation, we’d have to lie in the sun to control body temperature. Higher life forms, such as mammals, possess the unique ability to regulate body temperature, or thermoregulation.

Thermoregulation sets higher life forms apart from lower forms like reptiles that hark back to more ancient ancestors. While crocodiles and snakes bask in the sun to regulate body temperature, higher animals like humans control body temperature through a set of internal controls. This evolutionary advantage allows mammals to range more widely in the world and tolerate a greater range of temperature changes.

Should thermoregulation go haywire due to external influences that overwhelm it, trouble results. Normal responses keep temperature fluctuations contained within a few degrees. Violating the normal range up or down can lead to illness and death. Anyone who has experienced the few degrees higher temperature of a fever or the reduction of a few degrees of hypothermia from exposure know that just a few degrees in either direction is distinctly unpleasant, even life threatening. Allow a fever to exceed 105º F, and seizures and death can result. Low temperatures of 94º F are usually accompanied by shivering, followed by confusion; further reductions below 90º involve unconsciousness and shutting down of bodily functions.

But can less dramatic variations of temperature between 94º and 99º F signal subtle disruptions of thermoregulation that provide insight into health? That is the issue we wish to consider. In particular, we want to consider whether temperature assessment yields information into thyroid and metabolic status. Does this help us any way in our plaque-control program?

Circadian variation

Body temperature normally displays a predictable cycling pattern through the day and night, or circadian rhythm.

The typical temperature variation of humans shows the following pattern (rectal temperatures):




Fig 1. From Duffy JF et al 1998. Core (rectal) body temperatures for young and older subjects. Solid circles = Older subjects (n =43); open circles = young subjects (n = 97); solid bar, usual sleep episode of older subjects; open bar, usual sleep episode of young subjects. Data are plotted with respect to actual time of day.


From Figure 1, note that older people (over age 65; solid circles) show their lowest temperature around 2-3 a.m., 3 hours earlier than younger people (average age 23 years; open circles) (Duffy JF et al 1998). Also note that the lowest temperature (nadir) in older people is 97.61º F/36.45º C, while the lowest temperature in younger people is 97.25º F/36.25º C, nearly one-half Fahrenheit degree lower in the younger group. The difference between the height of temperature, which occurs in early evening, and the nadir is approximately 1.8º F/1º C.

(Importantly, these data, as well as the majority of other data examining human temperature variation are not corrected for thyroid status.)

Circadian variation is susceptible to numerous influences that can increase or decrease the high temperature reached, the dip of low temperature, and the timing of temperatures.

Among the most important influences on thermoregulation are:
 

• “Chronotype“: ”Morning people,” i.e., people who are most alert and productive in the morning, tend to have approximately 0.9º F/0.5º C lower a.m. nadir temperatures that also occur earlier as compared to “night people” (Waterhouse J et al 2001).
• Age: While people <65 years show their lowest temperature at 2-3 a.m., younger people do so around 6 a.m.; the age difference in fluctuations can be abolished through use of melatonin, though melatonin may also shift temperatures slightly lower in the elderly (0.2-0.5º F/0.1-0.3º C; Gubin DG et al 2006).
• Alcohol: While alcohol consumption generally exerts a slight overall temperature-reducing effect, it has also been shown to blunt the early a.m. nadir, thus yielding misleadingly high morning temperatures (Danel T et al 2001).
• Fasting: Both fasting and substantial calorie restriction reduce temperatures, likely a survival mechanism in response to reduced food availability (Kelly GS 2007).
• Sleep deprivation: While acute sleep deprivation increases temperatures, chronic sleep deprivation exerts an effect similar to that of fasting, i.e., reduction in temperature, including reduction of early a.m. nadir (Kelly GS 2007).
• Fitness: Interestingly, limited data suggest that very physically fit people tend to have lower early a.m. temperatures by 0.4º F/0.2º C (oral) than unfit people (Atkinson G et al 1993).
• “Weekend effect”: Temperatures tend to be higher on Saturday and Sunday if you sleep later when not working. Kelly (2007) suggests that the temperature can be expected to increase 0.18º F/0.1º C for every hour later you awake from habitual sleep times.
• Menstrual cycle: For women experiencing menses, temperatures (including a.m. nadir) are shifted higher 0.9º F/0.4º C starting 14 or so days after menstrual bleeding begins (the “luteal” phase, when progesterone levels are high); true low temperatures are therefore measured only during the first 7 days after onset of menstrual bleeding (“follicular” phase), when true temperature nadirs are experienced (Coyne MD et al 2000; Kelly G 2006). Temperatures taken during the follicular phase more closely mimic that of males (Baker WC et al 2001.) Women taking oral contraceptives or progesterone also track 0.9º F/0.4º C higher, with a loss of the menstrual cycling of temperatures (Baker FC et al 2001). Estrogen replacement has a small effect, shifting the temperature curve lower by 0.18º F/0.1º C (Gudmundsson A et al 1999).

Fig 2. From Baker FC et al 2001. Rectal temperatures in men and in healthy women by menstrual phase.  (http://jp.physoc.org/content/vol530/issue3/fulltext/565/Figures/565-F1.gif)

Thermoregulation is under the control of the hypothalamus. Protection from high temperatures, such as the several degree increase experienced during intensive exercise, is controlled via heat loss through increased blood flow to the skin and sweating. Protection from cooler ambient temperatures and maintenance of a constant internal temperature is also under the control of the hypothalamus, but heavily dependent on the healthy function of the thyroid.

Research on thermoregulation break temperature control into two categories: 1) obligatory, the maintenance of basal temperature control (basal metabolic rate, or BMR), that is under the control of the hypothalamus-pituitary-thyroid glands; and 2) facultative, the response to cooler temperatures, involving shivering, the state of skin blood vessel dilatation or constriction, and muscle activity, under the control of the hypothalamus and adrenal glands, as well as the sympathetic nervous system that produces catecholamines (e.g., adrenaline) (Silva JE 2003).

What is normal temperature? Recent analyses, as well as a comprehensive review of temperature data from studies from 1935 to 1998, suggests that normal oral temperature ranges from 96.3º F/35.7º C to 99.9º/37.7º C (Sund-Levander M et al 2002; Gomolin IH et al 2007; McGann KP et al 1993). This differs from the 98.6º F/37.0º C often quoted as normal, a relic of the original 19th century observations on human temperatures in health and disease.

Also note that many acute and chronic disease states such as infections, active allergies, depression, etc. can affect temperature.

(How about the effect of combinations of influences on temperature and circadian rhythm? For example, how about a fit, athletic female who drinks two glasses of wine per night and is a “night owl” who takes a combination estrogen/progestin birth control pill? Unfortunately, there are no such data to guide us.)

How should temperature be measured?

Internal organ temperatures best reflect body temperature. In research, temperatures from the pulmonary artery, gastrointestinal tract, bladder, urine, or rectum are used, though even rectal temperatures track slightly below that of true internal temperatures. However, for convenience, oral temperatures are often used, even though oral temperatures track approximately 1º F/0.55º C below that of internal temperature.

The traditional followers of the substantial, though anecdotal, experience of Dr. Broda Barnes, adhere to his original belief that axillary (armpit) temperatures are the preferred method to assess body temperature. Dr. Barnes correctly reasoned that measuring temperatures can provide insight into thyroid status. But how reliable are axillary temperatures? Of all the various ways to measure body temperature, axillary is the least reliable and the one most prone to inaccuracy. More so than other methods, axillary temperatures are subject to external ambient temperature, amount of clothing worn prior to temperature measurement, sweating, whether right or left arm is used (since there is variation of up to 2.0º F degrees from right to left), the amount of cutaneous (skin) dilation or constriction of blood vessels. Axillary temperatures track rectal temperature poorly, with wide variation in the day-to-day and minute-to-minute fluctuations of temperature, and especially marked divergence from rectal temperature in morning (temperature nadir) and evening (temperature peak) hours, with as much as 1.8-2.7º F/1.0-1.5º C variation within several minutes (Cattaneo CG et al 2000; Kelly G 2006). Axillary temperatures are therefore too variable and unreliable for use in assessing thermoregulation, whether for research or our purposes..

Thermoregulation, hypothyroidism, and “adrenal fatigue”

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