What does the thyroid do, and which nutrients does it need?

The thyroid's hormonal cascade works through a feedback loop. The hypothalamus releases TRH, which signals the pituitary to release TSH, which tells the thyroid to produce T4 and T3. About 80% of circulating T3 is actually produced outside the thyroid by converting T4 into T3 in the liver, kidneys, and other tissues². This conversion step is critical -- you can have a thyroid producing adequate T4 but still experience hypothyroid symptoms if conversion to T3 is impaired.

Hashimoto's thyroiditis accounts for roughly 90% of hypothyroid cases in iodine-sufficient countries¹. The immune system produces antibodies (TPO and thyroglobulin antibodies) that gradually destroy thyroid tissue. It often progresses slowly over years, which is why many people live with subclinical symptoms long before diagnosis.

Nutrient Cofactors

The thyroid depends on several key nutrients to produce and convert its hormones:

• Iodine is the raw material for thyroid hormones -- T4 contains four iodine atoms, T3 contains three. Without adequate iodine, the thyroid simply cannot produce enough hormone. (See the Iodine page for more detail.)²
• Selenium is required for the deiodinase enzymes that convert T4 to active T3. It also helps protect the thyroid from oxidative damage during hormone production. (See the Selenium page.)³
• Zinc supports TSH synthesis and T4 to T3 conversion. Zinc deficiency has been associated with altered thyroid hormone levels⁴.
• Iron is a cofactor for thyroid peroxidase (TPO), the enzyme that synthesizes thyroid hormones. Iron deficiency -- common in women -- can impair thyroid function independent of iodine status².

Goitrogens: Overstated Fear

Cruciferous vegetables like broccoli, kale, and cauliflower contain goitrogens -- compounds that can theoretically interfere with iodine uptake by the thyroid. In practice, this is not a meaningful concern unless you are already iodine-deficient and eating extremely large amounts of raw cruciferous vegetables². Cooking reduces goitrogen content significantly. The nutritional benefits of these vegetables far outweigh the negligible goitrogenic risk for most people.

Testing Beyond TSH

Standard thyroid screening often measures only TSH, but TSH alone can miss important dysfunction. A complete thyroid panel should include Free T4, Free T3, and thyroid antibodies (TPO-Ab and TG-Ab)¹. Someone with a "normal" TSH may still have low Free T3 due to poor conversion, or elevated antibodies indicating early Hashimoto's years before TSH becomes abnormal.

Hashimoto's Thyroiditis -- The Autoimmune Driver

Hashimoto's is characterized by lymphocytic infiltration of the thyroid gland and the presence of anti-thyroid peroxidase (TPO) and anti-thyroglobulin (TG) antibodies¹. Caturegli et al. (2014) reviewed the diagnostic criteria and found that TPO antibodies are present in approximately 95% of Hashimoto's cases, making them the most sensitive serological marker. The condition has a strong female predominance (7:1 female-to-male ratio) and a significant genetic component, with concordance rates of approximately 55% in monozygotic twins.

The progression from euthyroid Hashimoto's to overt hypothyroidism occurs at a rate of roughly 5% per year once antibodies are elevated¹. This slow progression means that antibody testing can identify at-risk individuals years before TSH rises above the reference range, offering a window for nutritional and lifestyle intervention.

Iodine and Selenium Interdependence

Zimmermann and Boelaert (2015) demonstrated that iodine and selenium status must be considered together². Iodine deficiency remains the most common cause of hypothyroidism globally, affecting nearly 2 billion people. However, selenium deficiency compounds the problem: the selenoenzyme glutathione peroxidase protects thyrocytes from hydrogen peroxide generated during iodine organification. Without adequate selenium, iodine supplementation can actually increase thyroid oxidative damage³.

Ventura et al. (2017) reviewed the evidence for selenium in thyroid disease and found that selenomethionine supplementation (200 mcg/day) consistently reduced TPO antibody titers in Hashimoto's patients across multiple RCTs³. The proposed mechanism involves both enhanced antioxidant protection via glutathione peroxidase and improved T4-to-T3 conversion via type 1 and type 2 deiodinase enzymes, both of which are selenoproteins.

Zinc and Thyroid Hormone Metabolism

Severo et al. (2019) reviewed the evidence that zinc plays a key role in thyroid hormone metabolism, regulating the activity of deiodinase enzymes, the synthesis of TRH and TSH, and the structure of transcription factors involved in thyroid hormone synthesis⁴. Serum zinc concentrations also appear to influence serum T3, T4, and TSH levels. Zinc transporters are present in the hypothalamus, pituitary, and thyroid, but their functions remain unknown, and the authors call for further investigation of zinc's role in thyroid hormone regulation.