Hypothyroidism
Underactive thyroid condition causing fatigue, weight gain, and metabolic slowdown.
TL;DR
Hypothyroidism is confirmed or excluded with a TSH blood test — full stop. Symptoms alone cannot diagnose it, and treating symptoms with thyroid hormone when tests are normal does not help and carries risk. Iodine is the wrong lever in developed countries, where deficiency is uncommon and excess iodine can trigger thyroid dysfunction. Selenium lowers thyroid antibody levels in Hashimoto's but has not convincingly improved symptoms. Levothyroxine works; take it fasting, away from calcium, iron and coffee.
Overview
Hypothyroidism — an underactive thyroid — is the thyroid's failure to produce enough hormone for the body's needs, slowing metabolism across every organ system. The result is a recognisable cluster: fatigue, weight gain, cold intolerance, constipation, dry skin, brain fog and low mood — symptoms so common and non-specific that hypothyroidism is simultaneously over-diagnosed by suspicion and under-diagnosed by testing. It is common, easily confirmed with a blood test, and straightforwardly treated with levothyroxine — but not a diagnosis to make or treat on symptoms alone.
Common Symptoms
- •Fatigue and low energy disproportionate to activity
- •Weight gain despite unchanged eating — typically modest, 2-5kg
- •Cold intolerance — feeling cold when others are comfortable
- •Constipation
- •Dry skin, coarse or thinning hair, brittle nails
- •Brain fog, slowed thinking and poor concentration
- •Low mood and depression
- •Heavy or irregular periods, reduced fertility
- •Hoarse voice, puffy face, slowed heart rate in more advanced disease
Common Causes
- •Hashimoto's thyroiditis — autoimmune destruction, the dominant cause
- •Thyroid surgery or radioactive iodine treatment
- •Radiation to the neck
- •Medications — lithium, amiodarone, immune checkpoint inhibitors, interferon
- •Iodine deficiency — the leading cause worldwide, uncommon where salt is iodised
- •Iodine excess — kelp and high-dose supplements can both cause and worsen hypothyroidism
- •Pituitary disease — rare central hypothyroidism
- •Postpartum thyroiditis — usually temporary
Root Causes
In Hashimoto's, the immune system generates antibodies against thyroid peroxidase and gradually destroys hormone-producing tissue — thyroid output declines over years, and TSH rises as the pituitary drives the failing gland harder. The symptoms are the metabolism-slowing consequences of insufficient hormone reaching every tissue: the brain slows (fog, low mood), the gut slows (constipation), heat production falls (cold intolerance), skin turnover drops (dry skin, hair changes). Because these are the most common symptoms in the general population, most people with them do not have hypothyroidism — which is exactly why the blood test, not the symptom list, makes the diagnosis.
How It's Diagnosed
Diagnostic Markers
- TSH — the screening test; high TSH with low free T4 confirms overt hypothyroidism
- High TSH with normal T4 is subclinical — treatment decisions are individualised
- TPO antibodies — confirm Hashimoto's and predict progression
- Repeat borderline results after 2-3 months before committing to treatment
- Once treated, TSH guides dose — recheck 6-8 weeks after any change
When to See a Doctor
{"A symptom cluster of fatigue, weight gain, cold intolerance and constipation — worth one TSH test","Symptoms with a family history of thyroid or autoimmune disease","Planning pregnancy or pregnant — thyroid control matters for the baby","New symptoms while on lithium or amiodarone","A lump in the neck or visible thyroid enlargement","Symptoms persisting despite a TSH in range on treatment — dose may need adjusting"}
Supplements Studied For This
Diet & Lifestyle
Suggested Pattern
Diet does not treat hypothyroidism, but it can sabotage its treatment: calcium, iron, soy, high-fibre meals and coffee all impair levothyroxine absorption if taken within hours of the dose. Selenium-rich foods (Brazil nuts — one or two a day is ample, seafood, eggs) are reasonable. Kelp and high-dose iodine supplements are not: they can trigger autoimmune thyroiditis and worsen hypothyroidism, and are a frequent hidden cause of erratic thyroid tests. Adequate protein and a Mediterranean-style pattern support the weight and energy issues that bother patients most.
Eat more
- Brazil nuts — one or two daily covers selenium needs
- Seafood and eggs — selenium, zinc and iodine at food-safe levels
- Adequate protein at each meal to support energy and weight
- Iodised salt at normal culinary levels
- Iron-rich foods — iron deficiency impairs thyroid hormone production, but take iron pills away from levothyroxine
Avoid
- Kelp, spirulina and high-dose iodine supplements — a genuine trigger of thyroid dysfunction
- Calcium and iron supplements within 4 hours of levothyroxine
- Soy and high-fibre meals immediately around the levothyroxine dose
- Coffee with the morning dose — wait 30-60 minutes
- Very high-dose biotin before thyroid blood tests — it distorts TSH results
Supporting Research
Hypothyroidism
Thyroid Hormone Therapy for Older Adults with Subclinical Hypothyroidism
Myo-inositol plus selenium supplementation restores euthyroid state in Hashimoto's patients with subclinical hypothyroidism
Excess iodine intake and thyroid dysfunction: a systematic review of population and clinical evidence
Thyroid dysfunction associated with kelp supplement use: case series and review
Frequently Asked Questions
Who It Affects
Overt hypothyroidism affects roughly 0.3-0.5% of the population, with subclinical disease in a further 3-8%. Women are affected five to ten times more often than men, and prevalence rises steeply with age.
Peak onset in middle age; Hashimoto's clusters in families and alongside other autoimmune conditions (coeliac, type 1 diabetes, pernicious anaemia). Postpartum thyroiditis affects 5-10% of women in the year after delivery. High iodine intake from kelp supplements is a genuine and growing trigger in supplement users.
Quick Facts
- •TSH is the test — symptoms alone cannot diagnose hypothyroidism
- •Affects roughly 5% of adults; women outnumber men 5-10 to 1
- •Hashimoto's autoimmune thyroiditis is the cause in most developed-world cases
- •Iodine supplements can trigger or worsen thyroid dysfunction — usually the wrong move
- •Selenium lowers antibody levels but has not convincingly improved symptoms
- •Levothyroxine must be taken fasting, 30-60 minutes before food, away from iron and calcium
Lifestyle Tips
- •Take levothyroxine fasting, 30-60 minutes before breakfast, with water
- •Keep timing and brand consistent — absorption varies
- •Regular aerobic and resistance exercise addresses fatigue and weight better than any supplement
- •Prioritise sleep — hypothyroid fatigue and poor sleep compound each other
- •Ask for a TSH before accepting that symptoms are just stress or ageing
- •If on treatment and still symptomatic with a normal TSH, look beyond the thyroid — iron, B12, sleep, mood
My Notes
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This information is for educational purposes only. Always consult a healthcare professional before starting any supplement regimen.