Thyroid, honestly — treatment, over-testing and the grey zone

Levothyroxine is life-changing for a genuinely underactive thyroid — but the evidence says most mild “subclinical” cases, especially in older adults, gain nothing from treatment, and both too little and too much iodine harm the thyroid.

What the thyroid does — and how it’s tested

Your thyroid sets your metabolic pace. Doctors screen it mainly with TSH, a pituitary signal that rises when the thyroid is underactive and falls when it’s overactive — so TSH moves opposite to your actual thyroid hormone. A high TSH with a low free-T4 is overt hypothyroidism; the messy middle — a high TSH with a still-normal T4 — is “subclinical”.

The most common cause where iodine intake is adequate is Hashimoto’s, an autoimmune attack on the gland.

What the evidence supports

Where treatment clearly earns its place.

• Levothyroxine works for a genuinely underactive thyroid — Synthetic T4 (levothyroxine) is the standard, effective treatment for overt hypothyroidism. Combination or “natural” thyroid preparations don’t reliably beat it. (established)
• Iodine is a U-shape: too little AND too much harm — Iodine deficiency causes thyroid disease — but so does excess. More iodine is not “more thyroid health”, and high-dose iodine or kelp supplements can trigger dysfunction. (established)

Where it’s over-treated and over-tested

The honest grey zone.

• Mild subclinical hypothyroidism usually needs no treatment — A large randomised trial (TRUST, 737 adults ≥65) lowered TSH with levothyroxine but improved neither symptoms nor tiredness versus placebo. Treating a slightly high TSH in someone who feels well mostly medicalises a number. (note)
• Screening everyone isn’t supported — Routine thyroid screening of symptom-free non-pregnant adults isn’t recommended (US preventive taskforce rates the evidence insufficient) — it turns up borderline numbers that lead to over-diagnosis. (note)
• Selenium for Hashimoto’s is oversold — Selenium may modestly lower thyroid antibodies in Hashimoto’s, but a Cochrane review found the evidence insufficient to show it actually helps you — it is not a cure, and antibody levels alone don’t require treatment. (hype)

What we still don’t know

• Who with subclinical hypothyroidism does benefit Younger people, very high TSH, or strong symptoms may differ from the older trial populations — this is genuinely unsettled and individual.
• Whether some people need a lower “optimal” TSH The popular idea that everyone feels best with a TSH under ~2.5 isn’t supported by treatment trials in people who feel well.

Key terms

• TSH A pituitary signal that rises when the thyroid is underactive; the main screening test. Moves opposite to thyroid hormone.
• Free T4 The active thyroid hormone in the blood; a low T4 with a high TSH means overt hypothyroidism.
• Hashimoto’s An autoimmune attack on the thyroid — the leading cause of an underactive thyroid where iodine is adequate.
• Subclinical hypothyroidism A high TSH with a still-normal T4, often without symptoms — the treatment grey zone.
• Levothyroxine Synthetic T4, the standard replacement for an underactive thyroid.
• TPO antibodies A marker of thyroid autoimmunity; raised levels alone don’t require treatment.