Question 1

How does the thyroid control metabolism and TDEE (Total Daily Energy Expenditure)?

Accepted Answer

The thyroid gland produces T4 (thyroxine) and T3 (triiodothyronine), which directly regulate basal metabolic rate (BMR)—the largest component of TDEE (60-70%). T3 is the active hormone that enters cell mitochondria and upregulates ATP production, oxygen consumption, and thermogenesis. When T3 levels drop, BMR can decrease by 15-40%, meaning fewer calories burned at rest. This explains why hypothyroid individuals often experience unexplained weight gain despite unchanged diet. T3 also regulates brown adipose tissue activation, gut motility (affecting nutrient absorption timing), and cardiac output—all of which influence total energy expenditure. Subclinical hypothyroidism (TSH 4.5-10 mIU/L) can reduce TDEE by 200-400 kcal/day without obvious symptoms.

Question 2

What are the basics of T3, T4, and TSH, and how do they interact?

Accepted Answer

TSH (Thyroid-Stimulating Hormone) is produced by the pituitary gland and acts as the 'thermostat' for thyroid function. When the hypothalamus detects low circulating thyroid hormones, it releases TRH, which triggers TSH release. TSH stimulates the thyroid to produce T4 (80%) and T3 (20%). T4 is the 'storage' hormone with a half-life of ~7 days; T3 is the active hormone (4x more potent) with a half-life of ~1 day. Most T3 is created by converting T4 via deiodinase enzymes in the liver, gut, and peripheral tissues. This conversion requires selenium, zinc, and iron. Key lab ranges: TSH 0.4-4.0 mIU/L (optimal 1.0-2.0), Free T4 0.8-1.8 ng/dL, Free T3 2.3-4.2 pg/mL. A high TSH with normal T4/T3 indicates subclinical hypothyroidism—the thyroid is struggling but compensating.

Question 3

How does stress (cortisol) suppress thyroid conversion and function?

Accepted Answer

Chronic stress elevates cortisol, which suppresses thyroid function through multiple pathways: (1) Cortisol inhibits TSH secretion from the pituitary, reducing the signal to produce thyroid hormones; (2) It impairs the conversion of T4 to active T3 by downregulating Type 1 deiodinase enzyme, while upregulating Type 3 deiodinase which converts T4 to reverse T3 (rT3)—a metabolically inactive form that blocks T3 receptors; (3) Cortisol increases thyroid-binding globulin (TBG), reducing free (available) T3 and T4; (4) It promotes intestinal permeability ('leaky gut'), triggering autoimmune thyroid inflammation (Hashimoto's pathway). This creates 'Low T3 Syndrome'—labs may show normal TSH and T4, but T3 is low and rT3 is high. Basal temperature drops below 36.4°C, metabolism slows, and symptoms mimic hypothyroidism despite 'normal' standard labs.

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