The thyroid gland produces thyroxine, T4, and triiodothyronine, T3, hormones that regulate the metabolic rate of virtually every cell in the body. They control the rate of protein synthesis, mitochondrial activity, heart rate, temperature regulation, digestion, and neural function. When thyroid hormone production falls below requirement, every metabolic process slows. The result is a cluster of symptoms that, because they are so pervasive and non-specific, are frequently misattributed to other causes: persistent fatigue, weight gain despite unchanged diet, cold intolerance, constipation, dry skin, hair loss, brain fog, depression, and muscle weakness.
The Reference Range Problem
TSH, thyroid stimulating hormone, is the primary diagnostic marker for thyroid function. The standard UK laboratory reference range extends to approximately 4.5 to 5.5 mIU per litre. However, several studies, including a large population study in Norway, have found that TSH above 2.5 mIU per litre is associated with a progressively increasing symptom burden in a proportion of adults, and that within the so-called normal range, higher TSH correlates with more thyroid-related symptoms. The upper limit of the reference range was established from a population that included a significant proportion of individuals with subclinical thyroid disease, which has inflated what is considered normal.
T4, T3, and Conversion
Standard NHS thyroid testing measures TSH and sometimes T4. What it does not routinely measure is free T3, the active form of thyroid hormone that actually enters cells. T4 must be converted to T3 by enzymes in peripheral tissues, and this conversion can be impaired by chronic stress, inflammation, low selenium, caloric restriction, and certain medications. Adults with normal T4 but impaired T4-to-T3 conversion will have normal TSH, normal T4, and persistently low T3 with all the symptoms of hypothyroidism. This pattern is not captured by standard testing.
Nutrition and Thyroid Function
Several nutritional factors are essential for thyroid hormone production and conversion. Iodine is required for thyroid hormone synthesis, and iodine deficiency is the most common cause of hypothyroidism worldwide. In the UK, iodine intake is declining due to reduced dairy consumption and plant-based dietary trends. Selenium is the cofactor for deiodinase enzymes that convert T4 to T3. Two to three Brazil nuts daily provides the daily selenium requirement. Zinc is required for TSH synthesis. Iron deficiency impairs thyroid peroxidase, the enzyme required for T4 and T3 synthesis.
- Request TSH, free T4, and free T3 testing if you have symptoms of hypothyroidism: standard TSH-only testing is insufficient
- Eat two to three Brazil nuts daily for selenium, the most direct nutritional intervention for T4-to-T3 conversion
- Ensure adequate iodine: dairy products, white fish, and shellfish are the primary dietary sources in the UK
- Address iron deficiency before concluding thyroid function is normal: iron deficiency impairs thyroid hormone synthesis
- Avoid chronic severe caloric restriction: it impairs T4-to-T3 conversion independently of thyroid disease
Sources
- Canaris, G.J., et al. (2000). The Colorado thyroid disease prevalence study. Archives of Internal Medicine, 160(4), 526-534.
- Biondi, B., & Cooper, D.S. (2008). The clinical significance of subclinical thyroid dysfunction. Endocrine Reviews, 29(1), 76-131.