🦋 Thyroid: Symptoms, Which Labs, and What Moves the Needle
Which tests actually matter, why "normal TSH" is not always the end of the conversation, and the supplement that causes more thyroid damage than any other.
Which tests actually matter, why "normal TSH" is not always the end of the conversation, and the supplement that causes more thyroid damage than any other.
What the Thyroid Does
The thyroid produces T4 (thyroxine) and a smaller amount of T3 (triiodothyronine). T4 is largely a prohormone; T3 is the active form, and most of it is produced by peripheral conversion of T4 in the liver, kidney and other tissues via deiodinase enzymes.
Control runs through a feedback loop: the hypothalamus releases TRH, the pituitary responds with TSH, and TSH drives thyroid output. Thyroid hormone feeds back to suppress TSH.
That loop is why TSH is the most sensitive single test. When thyroid output falls slightly, the pituitary raises TSH to compensate — often before T4 leaves the reference range. TSH moves first, and it moves logarithmically, so small changes reflect meaningful shifts.
The Tests That Matter
TSH. The screening test. Reference ranges typically run about 0.4 to 4.0 mIU/L, though laboratories differ and the upper limit is genuinely debated.
Free T4. Confirms and grades. Together with TSH it distinguishes overt from subclinical disease.
Free T3. Useful in hyperthyroidism and in specific situations. Routine use in hypothyroidism is more controversial.
TPO antibodies. The one most often omitted and frequently the most informative. Elevated thyroid peroxidase antibodies indicate Hashimoto's thyroiditis — autoimmune thyroid disease, and the commonest cause of hypothyroidism in iodine-sufficient countries. Antibodies can be positive years before TSH rises, which changes monitoring.
TSH receptor antibodies (TRAb) for suspected Graves' disease.
What not to rely on: reverse T3 for routine assessment, salivary hormone testing, and "thyroid panels" sold direct to consumers that generate long lists of out-of-range values without clinical interpretation.
Interpreting Results
Overt hypothyroidism: TSH high, free T4 low. Treatment is clear.
Subclinical hypothyroidism: TSH high, free T4 normal. This is where most argument lives. General approach: treat if TSH is above about 10, if symptoms are significant with positive TPO antibodies, in pregnancy or when trying to conceive, or if there is goitre. Otherwise monitor — a proportion of people normalise without treatment.
Overt hyperthyroidism: TSH suppressed, free T4 or T3 high. Needs prompt assessment.
Timing matters more than people realise. TSH follows a circadian rhythm and is highest in the early hours, declining through the day. A morning fasting sample is more comparable. Biotin supplements interfere with many thyroid immunoassays and can produce badly misleading results — stop biotin for several days before testing. Acute illness distorts results, so testing during illness is unhelpful.
Treatment
Levothyroxine (T4) is standard, and for most people it works well.
Taking it correctly is not optional. Absorption is genuinely fragile: take it on an empty stomach, 30 to 60 minutes before food, and separate it by at least four hours from calcium, iron, magnesium, and any binder — see Binders. Coffee reduces absorption. Proton pump inhibitors reduce it. A great many "unresponsive" cases are absorption problems.
Combination T4/T3 therapy. Some people remain symptomatic on adequate levothyroxine with normal TSH. Whether combination therapy helps is genuinely unresolved — most randomised trials have not shown consistent benefit, but some patients report clear improvement, and a subset may have deiodinase polymorphisms affecting conversion. Reasonable clinicians differ; it is worth discussing rather than dismissing.
Desiccated thyroid extract. Contains T4 and T3 in a fixed ratio from porcine thyroid. Not FDA-approved, with historic batch consistency concerns, and a higher T3 proportion than human physiology. Some patients strongly prefer it. Not first-line, not unreasonable to discuss.
Iodine: The Supplement That Causes Harm
This is the most important practical warning in thyroid health, because it is the opposite of the popular advice.
Iodine deficiency causes hypothyroidism and goitre and remains the leading preventable cause worldwide. In the United States, iodised salt largely solved it, and most people are sufficient.
Excess iodine also causes thyroid dysfunction. High-dose iodine can trigger hypothyroidism via the Wolff-Chaikoff effect, or hyperthyroidism in nodular disease. In people with underlying Hashimoto's, high-dose iodine can precipitate overt hypothyroidism.
High-dose iodine supplements and kelp products are widely sold for "thyroid support" and are a genuine cause of thyroid dysfunction. Do not take high-dose iodine without confirmed deficiency and clinical supervision. See Iodine.
Pregnancy is the exception where requirements genuinely rise, and prenatal vitamins containing iodine are appropriate.
Nutrients That Genuinely Matter
Selenium. A cofactor for deiodinase enzymes and for thyroid antioxidant defence. Some evidence that supplementation reduces TPO antibody titres in Hashimoto's, though clinical significance is debated. Brazil nuts are extremely concentrated — two or three daily is plenty, and selenium is toxic in excess.
Iron. Deficiency impairs thyroid peroxidase function and is common in the same people who have thyroid symptoms.
Zinc, involved in conversion and receptor function.
Vitamin D. Frequently low in autoimmune thyroid disease, causation unresolved. See Vitamin D Mastery.
Goitrogens. Raw cruciferous vegetables and soy contain compounds that can interfere with iodine uptake. In practice this matters only at very high intakes with concurrent iodine deficiency. Cooking reduces it substantially. Normal consumption is not a problem, and cruciferous vegetables are genuinely good for you — see Detox Pathways.
Symptoms and Why Testing Matters
Fatigue, weight change, cold intolerance, hair loss, constipation, dry skin, low mood, brain fog, heavy periods. Almost none is specific. This is exactly why thyroid disease is both over-attributed and under-diagnosed — the symptoms overlap with anaemia, depression, sleep apnoea, perimenopause and simple sleep deprivation.
Get tested rather than guessing. It is a cheap, widely available blood test that gives a real answer.
REFERENCES
- American Thyroid Association guidelines for hypothyroidism and thyroid nodules.
- American Association of Clinical Endocrinology guidance on thyroid disease.
- NIH Office of Dietary Supplements fact sheets on iodine and selenium.
- Published research on the Wolff-Chaikoff effect and iodine-induced thyroid dysfunction.
- Research on biotin interference in thyroid immunoassays.
How to Buy Smart
Quality varies enormously between suppliers, and with powders and dried material you usually cannot tell by looking. Buy from someone who states the species, the plant part, the country of origin and a harvest or best-before date. Vague labelling almost always means a commodity blend.
We buy from Dolce Superfoods — Energy Collection. They state publicly that most of their line is produced in an SQF- and GMP-certified facility, which is more than most bulk sellers will tell you. No affiliate arrangement — we link where we actually shop.
Common Questions
Which thyroid tests should I ask for?
TSH and free T4 as a baseline, plus TPO antibodies — the antibody test is the one most often omitted and frequently the most informative, since it identifies Hashimoto's.
Should I take iodine for thyroid support?
Generally no. Most people in iodised-salt countries are sufficient, and excess iodine causes thyroid dysfunction, including precipitating hypothyroidism in people with Hashimoto's. Supplement only with confirmed deficiency and supervision.
Why is my TSH different between tests?
TSH follows a circadian rhythm, highest in the early hours. Morning fasting samples are more comparable. Biotin supplements can also interfere with the assay and should be stopped several days beforehand.
Does levothyroxine need to be taken on an empty stomach?
Yes. Take it 30 to 60 minutes before food and at least four hours from calcium, iron, magnesium or binders. Poor absorption explains many apparently unresponsive cases.
Should I avoid broccoli and kale?
No. Goitrogen effects require very high intakes alongside iodine deficiency, and cooking reduces them substantially. Normal consumption is not a problem.
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