Cortisol
Produced by: Adrenal gland.
Definition / Purpose: Regarding the thyroid gland, Cortisol decreases TSH, lowering thyroid hormone production. Cortisol also inhibits the conversion of T4 to T3, and increases the conversion of T4 to reverse T3.
Lab Reference Range: Dependent on test ordered. See below.
24 hour Urinary Free Cortisol Level: 4.0 – 50.0 mcg/24 hrs

AM Cortisol: (7 - 9AM): 4 – 22 mcg/dL

Circadian Rhythm Salivary Free Cortisol Levels:
7-9 AM: 0.27-1.18 mcg/dL
11-1 PM: 0.10-0.41 mcg/dL
3-5 PM: 0.05-0.27 mcg/dL
10-12AM: 0.03-0.14 mcg/dL
When to order:

• Screening test that should be ordered when looking for thyroid disease.
• Follow up lab testing to confirm normal levels.

Pearls:
Half-life: 1-2 hours
Symptoms of low cortisol level are very similar to hypothyroidism.
HBA1C levels below 5.3% may indirectly represent low cortisol levels.
If you have low cortisol levels (AKA adrenal fatigue), starting thyroid hormone replacement therapy (THRT) may worsen the adrenal fatigue and actually cause a worsening of your hypothyroid symptoms. Why? THRT has a tendency to increase the metabolism of your body’s cortisol causing lower levels. Such a person may not be able to tolerate THRT unless they are also on low dose cortisol therapy or some type of nutritional adrenal support.