Basal temperature

Basal temperature is morning body temperature measured before getting out of bed. Broda Barnes used it as a clinical sign of thyroid activity; Ray Peat pairs it with resting pulse because temperature alone is ambiguous.

The combination of pulse rate and temperature is much better than either one alone... Healthy and intelligent groups of people have been found to have an average resting pulse rate of 85/minute, while less healthy groups average close to 70/minute.

— Ray Peat

Practice

How to measure

Use a digital thermometer accurate to 0.1°F; the cheap 0.2°F models are not precise enough to catch the changes that matter.

1. Morning baseline. On waking, before drinking water, eating, or getting out of bed, take oral temperature and count pulse at the wrist for a full 60 seconds.
2. Post-breakfast reading. 30-60 minutes after a full breakfast (protein, carbohydrate, some fat), repeat both measurements. Eating is thermogenic and normally raises both temperature and pulse.
3. Afternoon reading. Between 3 and 4 PM, take a third reading; this is usually the daily peak.
4. Duration. Track at least two weeks before drawing conclusions, since that's roughly the half-life of thyroid hormone in the body and the window needed for a thyroid supplement to fully register.

Reading the pattern

The relationship between the morning and post-breakfast readings, not either number alone, is diagnostic.

Since food, especially carbohydrate and protein, will increase blood sugar and T3 production, eating is "thermogenic," and the oral (or eardrum) temperature is likely to rise after eating... In a normal person, both temperature and pulse rate rise after breakfast, but in very hypothyroid people either, or both, might fall.

— Ray Peat

The mechanism behind a high morning reading that then falls: blood sugar drops overnight, so adrenaline and cortisol rise to compensate, peaking around 1-2 AM and dawn respectively, artificially propping up morning temperature. Breakfast lets blood sugar (and T3) take over the job from the stress hormones, so the reading can fall even though the body is doing the right thing. A morning reading that is already low and falls further after breakfast is the more concerning pattern: the stress-hormone compensation itself is failing.

Confounders

Ignore or annotate readings on days when any of these apply, per Peat's own practice:

• Electric blankets and hot bedrooms. Peat began asking patients whether they used electric blankets, and discarded their morning data if they did.
• Ambient temperature. A hot, humid environment lets the body maintain 98.6°F with very little metabolic effort, masking hypothyroidism that would otherwise show up as low temperature; Peat found this in Eugene summers, where pulse rate (not temperature) revealed the problem.
• Menstrual cycle. Progesterone raises temperature in the luteal phase; track the two phases separately and use the follicular phase for a hypothyroid read. See Progesterone.
• Illness, alcohol, and hard exercise all shift readings independent of baseline thyroid status; note them rather than discard the day outright.

The Achilles reflex

A quick equipment-free check: with the foot hanging free, tap the Achilles tendon. In a euthyroid person the foot swings down and returns to neutral quickly. In hypothyroidism, the return to neutral is slow, "like a door-closer with worn fluid"; the slow relaxation, not the strength of the kick, is the hypothyroid sign.

Starting thyroid supplementation

If diet correction (salt, sugar, protein at regular intervals) doesn't move a flat, low morning reading over two weeks, thyroid supplementation may be needed. Start low: Peat described two patients who reacted with heart pain to as little as 15 mg of Armour, and traced it to magnesium deficiency in muscle tissue unable to buffer the sudden metabolic increase.

I have seen two people who reacted very surprisingly to 15 milligrams, a fourth of a grain, with heart pain, and I realized what's happening is that your cells, especially your big muscle cells...

— Ray Peat

See Magnesium before starting a thyroid supplement if a low-magnesium diet is likely.

Community targets often cite roughly 36.6 to 37.0 °C (97.8 to 98.6 °F) waking oral temperature, but Peat stresses context over a single number. Full workflow: Roadmap/02 - Self diagnosis & markers. Related: Hypothyroidism, Thyroid, Broda Barnes, Harm reduction.