Thyroid

Why thyroid function matters for longevity

Your thyroid gland sits at the base of your neck and produces two hormones, T4 (thyroxine) and T3 (triiodothyronine), that control how fast every cell in your body burns energy. When thyroid output drops even slightly, the effects ripple outward: metabolism slows, cholesterol rises, body temperature falls, cognition gets foggy, and cardiovascular risk climbs. A 2024 study in PNAS found that T3 levels are inversely related to mortality risk, while higher free T4 is positively associated with death, suggesting that the balance between these hormones matters more than either one alone ^[1].

Research on centenarians consistently shows a pattern: long-lived people tend to have mildly lower thyroid hormone levels and slightly higher TSH than younger adults. A 2023 review in Nature Reviews Endocrinology confirmed that healthy aging favors a "low-thyroid" phenotype, with reduced T3 and preserved TSH signaling ^[2]. This doesn't mean hypothyroidism is good. It means the thyroid axis recalibrates with age, and applying young-adult reference ranges to older people can lead to overdiagnosis and unnecessary treatment.

How your thyroid actually works

The thyroid gland takes iodine from your blood and combines it with the amino acid tyrosine to build T4 and T3. About 80% of what it releases is T4, a relatively inactive prohormone. Your liver, kidneys, muscles, and brain then convert T4 into T3 using enzymes called deiodinases that depend on selenium. T3 is the active form. It binds to receptors inside cell nuclei and directly controls gene expression for metabolic rate, heart function, body temperature, and even mood.

This conversion step is where many thyroid problems hide. Someone with "normal" TSH and T4 can still feel terrible if their T4-to-T3 conversion is poor due to selenium deficiency, chronic inflammation, or high cortisol. That's why a full thyroid panel, not just TSH, gives a more accurate picture.

Testing thyroid function properly

Most standard check-ups measure only TSH. That catches severe hypothyroidism but misses subtler dysfunction. A more complete panel includes:

TSH (0.4-4.0 mIU/L is standard, though many endocrinologists prefer 0.5-2.5 for optimal function), free T4, free T3, TPO antibodies (elevated in Hashimoto's thyroiditis, the most common cause of hypothyroidism), and thyroglobulin antibodies. Some practitioners also check reverse T3, though professional guidelines don't currently recommend routine rT3 testing because its clinical value outside of critical illness is debated ^[3].

Subclinical hypothyroidism, where TSH is elevated but T4 stays normal, affects 4-20% of the population depending on age and sex. It's most common in women over 60, where prevalence reaches about 15% ^[4]. Treatment with levothyroxine is generally recommended when TSH exceeds 10 mIU/L or TPO antibodies are elevated. For mild cases (TSH 4-10), the evidence for treatment benefit is weaker, especially in adults over 65.

Hashimoto's thyroiditis and the autoimmune connection

Hashimoto's thyroiditis is the leading cause of hypothyroidism in developed countries. The immune system attacks the thyroid gland, gradually destroying its ability to produce hormones. It affects women 7-10 times more often than men, and about 5% of the general population has it ^[5].

The gut-thyroid axis is an active area of research. A 2024 review found that people with Hashimoto's have reduced Lactobacillus and Bifidobacterium in their gut and increased harmful bacteria like Bacteroides fragilis ^[6]. Increased intestinal permeability ("leaky gut") may trigger or worsen thyroid autoimmunity. There's also a link to celiac disease: 2-9% of Hashimoto's patients have concurrent celiac, and a 2023 meta-analysis found that a gluten-free diet reduced TPO antibodies in some patients even without diagnosed celiac disease, though the evidence isn't strong enough yet for universal dietary recommendations ^[7].

Supporting thyroid health through nutrition and lifestyle

Iodine is the raw material for thyroid hormones. Adults need about 150 mcg daily, easily met through iodized salt, seafood, and dairy. Too much iodine can paradoxically worsen thyroid function, especially in people with existing autoimmune thyroiditis, so megadose supplements are a bad idea.

Selenium is just as important. It's required for the deiodinase enzymes that convert T4 to T3 and for the glutathione peroxidases that protect thyroid cells from oxidative damage. A 2024 meta-analysis of randomized controlled trials found that selenium supplementation (typically 200 mcg/day) significantly reduced TPO antibodies in Hashimoto's patients at both 3 and 6 months, and lowered TSH after 6 months ^[8]. Brazil nuts, seafood, and organ meats are the best food sources.

Exercise also helps. A meta-analysis of seven RCTs showed that long-term exercise reduced TSH and increased T4 in hypothyroid patients ^[9]. Chronic stress works in the opposite direction: elevated cortisol suppresses TSH release and impairs T4-to-T3 conversion, creating a functional thyroid deficit even when the gland itself is healthy.