Thyroid: Why TSH Alone Misses Most of the Problem

The standard thyroid screen is a single number: TSH, the pituitary's instruction to the thyroid gland. If it is in range, the investigation usually stops. But TSH is a signal about the gland, not about whether active hormone is reaching tissue.

The chain, and where it breaks

• The pituitary releases TSH
• The thyroid makes mostly T4, which is largely inactive
• Deiodinase enzymes in liver, kidney and other tissue convert T4 to active T3
• T3 enters cells and acts on receptors

TSH tells you about the first two steps. Symptoms come from the last two. Conversion is where a great many people run into trouble, and it is where nothing in the standard screen looks.

A panel worth asking for

• TSH, plus free T4 and free T3 — the free fractions, not total
• Reverse T3, which rises under stress, illness and severe caloric restriction and blocks the receptor without activating it
• TPO and thyroglobulin antibodies, because autoimmune thyroiditis is by far the most common cause of hypothyroidism in iodine-sufficient countries and is often present for years before TSH moves

What conversion depends on

• Selenium — the deiodinase enzymes are selenoproteins and cannot work without it
• Zinc and iron, both required at different points
• Adequate calories and carbohydrate. Prolonged severe restriction downregulates T3 deliberately; this is adaptation, not disease, but it produces the same symptoms
• Cortisol. Chronic stress shifts conversion toward reverse T3
• Liver function, since a large share of conversion happens there

A normal TSH with clear hypothyroid symptoms is not a reason to stop looking. It is a reason to look at the next step in the chain.

The autoimmune majority

Hashimoto's thyroiditis is an immune condition that happens to destroy the thyroid. Treating it purely as a hormone deficiency addresses the consequence and leaves the process running. Where it is present, the questions worth asking are about gut permeability, gluten, vitamin D status, selenium, and Epstein-Barr history — all areas where the research is suggestive and the interventions are low-risk.

None of this is a reason to refuse thyroid hormone if you need it. It is a reason not to treat the prescription as the end of the investigation. If you are being treated and still feel unwell, that is information, not a personality trait.