The thyroid gland produces hormones that regulate the metabolic rate of virtually every cell in the body. Thyroid hormone controls how fast cells burn fuel, how quickly the heart beats, how warm the body runs, how efficiently nutrients are converted into energy, and how quickly or slowly muscle protein is synthesised and broken down. When thyroid function declines, as it does progressively with age in a significant proportion of the population, the downstream effects are wide-ranging and often misattributed to other causes.
The Difference Between T4 and T3
The thyroid primarily produces thyroxine, known as T4, which is a relatively inactive precursor. The body converts T4 to the active hormone triiodothyronine, known as T3, primarily in the liver and gut. Many people with apparently normal TSH and T4 levels have impaired T4-to-T3 conversion, producing insufficient active hormone at the cellular level. Standard thyroid panels often measure only TSH and sometimes T4, missing the conversion problem entirely. This is one of the most common sources of ongoing symptoms in people who have been told their thyroid is normal.
Symptoms That Are Frequently Missed
- Persistent fatigue that does not respond to sleep or rest
- Weight gain that does not respond to dietary changes
- Cold intolerance and low body temperature
- Hair thinning, particularly at the outer third of the eyebrows
- Constipation and slow gut motility
- Brain fog and slowed processing speed
- Elevated cholesterol, particularly LDL, despite a clean diet
- Low mood and reduced motivation that does not respond to lifestyle changes
What Supports Thyroid Function
Iodine and selenium are the primary nutritional requirements for thyroid hormone synthesis and conversion. Two Brazil nuts per day provides approximately the daily selenium requirement. Iodine is found primarily in seafood and dairy. Chronic caloric restriction impairs T4-to-T3 conversion, as does high stress and elevated cortisol. A full thyroid panel including free T3 and thyroid antibodies provides a more complete clinical picture than TSH alone.
Sources
- Garber, J.R., et al. (2012). Clinical practice guidelines for hypothyroidism in adults. Thyroid, 22(12), 1200-1235.
- Biondi, B., & Cooper, D.S. (2008). The clinical significance of subclinical thyroid dysfunction. Endocrine Reviews, 29(1), 76-131.
- Hollowell, J.G., et al. (2002). Serum TSH, T4, and thyroid antibodies in the United States population. Journal of Clinical Endocrinology and Metabolism, 87(2), 489-499.