Condition Guide

Thyroid Disease

Name: Thyroid Disease: Hypothyroidism & Hyperthyroidism
Creator: Suman Konda
Published: 2026-07-05

The thyroid controls your metabolism, energy, weight and mood. Learn the difference between an underactive and overactive thyroid: and what your TSH result means.¹

Written by Suman Konda, Clinical Pharmacist · Based on peer-reviewed sources · Editorial policy · Not medical advice

Last reviewed and updated: 28 September 2026 · How we check our content

What thyroid disease is

The thyroid is a butterfly-shaped gland in the neck producing primarily thyroxine (T4) and triiodothyronine (T3) — hormones that regulate nearly every cell: metabolic rate, heart rate, body temperature, weight, mood, digestion, bone health and reproduction. When it makes too little or too much, virtually every body system is affected.

Disease broadly divides into two types. Hypothyroidism (underactive): high TSH with low T4, most commonly caused by Hashimoto’s thyroiditis — causing fatigue, weight gain, cold intolerance, constipation and low mood. Hyperthyroidism (overactive): suppressed TSH with high T4/T3, most commonly Graves’ disease — causing weight loss, palpitations, tremor, heat intolerance and anxiety. Both are far more common in women and become more likely with age.¹

Thyroid disease: myths vs facts

Thyroid folklore is persistent. Here is what the evidence says:

MYTH A slightly abnormal TSH always needs treatment.

FACT Borderline results are often repeated first, since a mildly abnormal TSH can be temporary. In subclinical hypothyroidism — raised TSH, normal free T4 — a trial of 737 adults aged 65 and over found levothyroxine normalised TSH but produced no improvement in symptoms or tiredness. A mildly raised TSH alone is not automatically a reason to start treatment, particularly in older people.²

MYTH Thyroid disease always causes big weight changes.

FACT The effect on weight is usually modest, and other factors matter too. Attributing all weight change to the thyroid misses the fuller picture.

MYTH You must fast before thyroid blood tests.

FACT No fasting is required. What helps is consistency — the same time of day when monitoring — and mentioning biotin supplements, which can distort results.

MYTH All thyroid problems are lifelong.

FACT Autoimmune hypothyroidism usually needs lifelong treatment, but thyroiditis after pregnancy or a viral illness can be temporary. The course depends on the cause, not the gland alone.

MYTH A normal TSH rules out all thyroid problems.

FACT Rare central hypothyroidism shows a low or normal TSH with low T4 — reading TSH together with free T4 is what maps the full picture. And thyroid nodules need ultrasound assessment regardless of blood results: nodules are very common (around a third of people in their 50s) and mostly benign, but suspicious features warrant fine-needle aspiration.³

What to do next

Thyroid workup follows a logical sequence:

Start with TSH, confirm with free T4. TSH is the single best initial test: high suggests underactivity, suppressed suggests overactivity. Free T4 (and sometimes free T3) confirms the pattern.
Identify the autoimmune cause. Anti-TPO antibodies point to Hashimoto’s thyroiditis; TSH-receptor antibodies to Graves’ disease — the distinction guides long-term management.
Repeat borderline results before acting. A mildly abnormal TSH can be temporary; treatment decisions should not rest on a single borderline value.
Get nodules imaged, not just blood-tested. Large nodules or those with suspicious ultrasound features may need fine-needle aspiration to rule out thyroid cancer — relatively rare but highly treatable when caught early.
Understand the treatment landscape. An underactive thyroid is treated with levothyroxine, adjusted using TSH and usually lifelong; an overactive thyroid with anti-thyroid medicines, radioactive iodine or surgery depending on the cause — with ongoing monitoring either way. Ask your doctor what TSH level to aim for on treatment.

ICD-10-CM diagnosis codes

Thyroid disorders are coded by whether the gland is under- or overactive and by the underlying cause:

ICD-10-CM code	Description
E03.9	Hypothyroidism, unspecified
E03.8	Other specified hypothyroidism
E03.1	Congenital hypothyroidism without goiter
E03.0	Congenital hypothyroidism with diffuse goiter
E89.0	Postprocedural hypothyroidism
E05.90	Thyrotoxicosis, unspecified, without thyrotoxic crisis

Codes shown are from the current ICD-10-CM classification (FY2026) and are provided for general reference. Clinical coding is performed by trained coders using the full medical record.

Practical notes

Thyroid management runs on trends, not single values. TSH can wobble with illness, time of day and assay differences, so comparing results from the same laboratory — ideally drawn at a similar time of day — gives the clearest picture. Keep every report with its date, and alongside the numbers note symptoms such as energy levels, weight changes, heat or cold sensitivity, and bowel habits, so your doctor can match the labs to how you actually feel.

A few details are disproportionately important for thyroid interpretation: exactly which thyroid medicine and dose you take and when (including the brand, since switching matters to some doctors), biotin-containing multivitamins or hair-and-nail supplements — biotin interferes with thyroid immunoassays and can fake abnormal results — calcium or iron supplements taken near the dose, recent pregnancy or illness, and family history of thyroid or autoimmune disease.

The practical habits doctors usually discuss are taking thyroid medicine consistently — the same way each day, as your doctor advised — separating calcium and iron supplements from the dose by the interval your pharmacist suggests, telling the lab about any biotin-containing supplements before a blood draw, and keeping iodine intake steady rather than extreme. Restrictive diets marketed online for thyroid disease are worth discussing with your doctor before trying.

After a dose change, TSH is typically rechecked in about six to eight weeks — earlier testing doesn't reflect the new steady state. Once stable, most people have thyroid function checked every 6–12 months, more often during pregnancy, when planning pregnancy, or after starting medicines known to affect the thyroid.

In India

A TSH test typically costs ₹250–₹600, a full thyroid panel (TSH with free T4, sometimes free T3) around ₹500–₹1,500, and anti-TPO antibody testing a few hundred rupees more — though prices vary by city and lab.

Look for NABL-accredited laboratories and imaging centres, which follow standardised quality processes. In major cities, most large labs offer home sample collection for the blood tests. Whatever a lab's website says about normal values, the reference range printed on your own report is the one that counts, since ranges differ between machines and assay methods.

Frequently asked questions

Which test is used to diagnose thyroid disease?

TSH is the main screening test, confirmed with free T4; antibody tests identify the underlying autoimmune cause — anti-TPO for Hashimoto’s thyroiditis, TSH-receptor antibodies for Graves’ disease. Because a mildly abnormal TSH can be temporary, borderline results are often repeated before treatment is started.

Are thyroid problems lifelong?

Autoimmune hypothyroidism usually needs lifelong treatment, while some thyroid conditions — such as after pregnancy or a viral thyroiditis — can be temporary. An overactive thyroid may resolve or need definitive treatment depending on the cause.

Can thyroid disease affect weight?

Yes. An underactive thyroid tends to cause weight gain and an overactive one weight loss, though the effect is usually modest and other factors matter too.

Do I need to fast for thyroid tests?

No fasting is required, but a consistent time of day helps when monitoring, and biotin supplements should be mentioned as they can affect results.

What do anti-TPO antibodies mean?

High anti-TPO (thyroid peroxidase) antibodies indicate Hashimoto’s thyroiditis — the commonest cause of hypothyroidism — where the immune system attacks the thyroid gland. They identify the autoimmune cause behind an underactive thyroid.

My TSH is slightly high but my T4 is normal — what does that mean?

This pattern is often called subclinical hypothyroidism — the pituitary is working a bit harder to keep thyroid hormone normal. Whether it needs treatment depends on how high the TSH is, your symptoms, antibody results and circumstances such as pregnancy plans, so it is an interpretation to make with your doctor rather than from the numbers alone.

Can the biotin in my multivitamin affect my thyroid test results?

Yes — biotin interferes with the immunoassay methods most labs use, and can falsely lower TSH or falsely raise thyroid hormone readings, mimicking disease that isn't there. Labs usually advise stopping biotin-containing supplements for a couple of days before the blood draw; tell the lab what you take so they can advise.

Should thyroid blood tests always be done at the same time of day?

It helps. TSH follows a daily rhythm — highest overnight, lower in the afternoon — so a morning sample compared against an afternoon one can look like a change that isn't real. For tracking trends, morning testing at a consistent time is the usual advice; ask your lab if they have a preferred slot.

Can thyroid problems affect pregnancy?

Yes — both underactive and overactive thyroid function are linked to pregnancy complications, and thyroid hormone needs often rise during pregnancy, so women on thyroid medicine usually need dose review and more frequent testing. If you are planning pregnancy or newly pregnant with a thyroid condition, discuss testing and monitoring with your doctor promptly.

References

Sources cited on this page. PubMed links open the original abstract.

Garber JR, Cobin RH, Gharib H, et al. Clinical practice guidelines for hypothyroidism in adults. Endocr Pract. 2012;18(6):988–1028. PMID 23246686 · doi:10.4158/EP12280.GL
Stott DJ, Rodondi N, Kearney PM, et al. Thyroid Hormone Therapy for Older Adults with Subclinical Hypothyroidism. N Engl J Med. 2017;376(26):2534–2544. PMID 28402245 · doi:10.1056/NEJMoa1603825
Mu C, Ming X, Tian Y, et al. Mapping global epidemiology of thyroid nodules among general population: A systematic review and meta-analysis. Front Oncol. 2022;12:1029926. PMID 36439513 · doi:10.3389/fonc.2022.1029926

Medical Disclaimer: This page is for general educational purposes only and does not constitute medical advice. Always consult a qualified doctor for diagnosis and treatment decisions.