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.

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

Last updated: 01 August 2026 · How we check our content

What does the thyroid do?

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

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How TSH and free T4 combine to give a thyroid diagnosis.

What the grid shows. TSH and free T4 are interpreted together. A high TSH with a low free T4 is overt hypothyroidism; a high TSH with a normal free T4 is subclinical hypothyroidism, which is the commonest abnormal pattern and often needs monitoring rather than treatment. A low TSH with a high free T4 is overt hyperthyroidism, and a low TSH with a normal free T4 is the subclinical form. The one combination that points away from the thyroid itself is a low TSH with a low free T4, which suggests a pituitary cause. The counter-intuitive part is that a high TSH means an underactive gland, because TSH is the signal telling the thyroid to work harder.

Thyroid blood test normal ranges

Test	Normal Range	Interpretation
TSH	0.4–4.0 mIU/L	The primary screening test: HIGH TSH = underactive
Free T4 (FT4)	0.8–1.8 ng/dL	LOW T4 + HIGH TSH = hypothyroidism
Free T3 (FT3)	2.3–4.1 pg/mL	Low in hypothyroidism, high in hyperthyroidism
Anti-TPO antibodies	<35 IU/mL	High = Hashimoto's thyroiditis (autoimmune)
Anti-TSH receptor Ab	Negative	Positive = Graves' disease (autoimmune hyperthyroidism)

Hypothyroidism: underactive thyroid

UNDERACTIVE High TSH + Low T4

Hypothyroidism means the thyroid isn't making enough hormone. Your pituitary gland raises TSH to try to stimulate the thyroid: so a high TSH is the earliest sign. The most common cause worldwide is Hashimoto's thyroiditis (autoimmune). Other causes include iodine deficiency, thyroid surgery, and radiation treatment. Subclinical hypothyroidism is when TSH is high but T4 is still normal, the thyroid is struggling but compensating.

Symptoms of hypothyroidism

Unexplained weight gain despite no change in diet
Persistent fatigue and sluggishness
Feeling cold all the time
Hair thinning or hair loss
Dry skin and brittle nails
Constipation
Depression and slow thinking ("brain fog")
Heavy or irregular menstrual periods
Elevated cholesterol (thyroid regulates lipid metabolism)

Hyperthyroidism: overactive thyroid

OVERACTIVE Low TSH + High T4/T3

Hyperthyroidism means the thyroid is producing too much hormone. TSH is suppressed (very low) because the pituitary is trying to slow the thyroid down. The most common cause is Graves' disease (an autoimmune condition where antibodies stimulate the thyroid). Other causes include toxic multinodular goitre, thyroid nodules, and thyroiditis (temporary inflammation releasing stored hormone).

Symptoms of hyperthyroidism

Unintentional weight loss despite increased appetite
Rapid or irregular heartbeat (palpitations)
Tremor (shaking hands)
Feeling hot and sweating excessively
Anxiety, nervousness, irritability
Diarrhoea or frequent bowel movements
Difficulty sleeping
Bulging eyes (exophthalmos): specific to Graves' disease
Muscle weakness

Thyroid nodules and goitre

A goitre is an enlarged thyroid gland: visible as a swelling at the base of the neck. It can occur with hypothyroidism, hyperthyroidism or even a normally functioning thyroid. Thyroid nodules (lumps in the thyroid) are very common, about 50% of people over 50 have them. Most are benign. Large nodules or those with suspicious features on ultrasound may need a fine needle aspiration biopsy (FNAC) to rule out thyroid cancer, which is relatively rare but highly treatable when caught early.

Treatment overview

Condition	Common Treatment
Hypothyroidism	Daily levothyroxine (synthetic T4) tablet: usually lifelong
Subclinical hypothyroidism	Monitor or treat if TSH >10 or symptoms present
Hyperthyroidism (Graves')	Antithyroid drugs (methimazole/carbimazole), radioactive iodine, or surgery
Hashimoto's	Levothyroxine if hypothyroid; monitor if euthyroid

Questions to ask your doctor

Should I check my Anti-TPO antibodies?
What TSH level should I aim for on treatment?
Do I need a thyroid ultrasound?
How long before I feel better on levothyroxine?
Does my diet (iodine, soy, gluten) affect my thyroid?

The two main types of thyroid disease

Thyroid disease broadly divides into an underactive thyroid (hypothyroidism), where too little hormone is made, and an overactive thyroid (hyperthyroidism), where too much is made. Each produces almost opposite symptoms: an underactive thyroid causes fatigue, weight gain, cold intolerance, constipation and low mood, while an overactive thyroid causes weight loss, palpitations, anxiety, heat intolerance and loose stools.

Both are far more common in women and become more likely with age.

How thyroid disease is diagnosed

Diagnosis rests on blood tests: primarily TSH, with free T4 (and sometimes free T3) to confirm. Thyroid antibody tests (such as anti-TPO) identify the autoimmune conditions that cause most thyroid disease: Hashimoto's thyroiditis for an underactive thyroid and Graves' disease for an overactive one.

Because a mildly abnormal TSH can be temporary, borderline results are often repeated before treatment is started.

Treatment and monitoring

An underactive thyroid is treated with levothyroxine, a hormone replacement taken once daily, with the dose adjusted using TSH. An overactive thyroid is treated with anti-thyroid medicines, radioactive iodine or surgery, depending on the cause and circumstances.

Both require ongoing monitoring: an underactive thyroid usually needs lifelong treatment, while some overactive thyroid conditions can settle or be cured.

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.

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.

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.

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.

References

The clinical information on this page is based on peer-reviewed sources indexed in PubMed, the biomedical literature database of the US National Library of Medicine.

Koulouri O, Moran C, Halsall D, Chatterjee K, Gurnell M. Pitfalls in the measurement and interpretation of thyroid function tests. Best Pract Res Clin Endocrinol Metab. 2013;27(6):745-62. doi:10.1016/j.beem.2013.10.003 · PMID 24275187
Pearce EN. Management of hypothyroidism and hypothyroxinemia during pregnancy. Endocr Pract. 2022;28(7):711-718. doi:10.1016/j.eprac.2022.05.004 · PMID 35569735