Your thyroid is a small, butterfly-shaped gland at the front of your neck. It makes thyroid hormone, which sets how fast your cells use energy, make body heat, and use fat, sugar and protein. Almost every part of your body depends on it, including your brain, heart, bones and muscles.^1,4

How it works, in three steps

1. Your brain sends a signal. A small gland under your brain, the pituitary, releases TSH (thyroid-stimulating hormone). TSH tells your thyroid to make more hormone. When your thyroid hormone falls, TSH rises; when there is enough, TSH falls back. This is why TSH is the most sensitive early sign that your thyroid is running low.^2,3
2. Your thyroid makes T4. Your thyroid uses iodine from food to make mainly T4 (thyroxine), the storage form of the hormone.¹
3. Your body turns T4 into T3. Your liver, kidneys, muscles, brain and other organs turn T4 into T3, the active form. T3 works inside your cells by switching genes on and off.^2,13

Almost all thyroid hormone in your blood is carried by proteins. Only the small “free” part can enter your cells and work, so we measure the free hormone, not the total.⁴

Changes with age. TSH tends to rise slowly as we get older, and the body clears thyroid hormone more slowly.⁵ Physicians who focus on optimal hormone levels also note that T4 and T3 fall gradually over the adult years.

Medical guidelines advise checking your thyroid if any of these apply to you:³

• You or a close relative has an autoimmune condition, such as type 1 diabetes or autoimmune thyroid disease
• You have had surgery or radiation to your neck, or radioactive iodine treatment, or you have a goitre (a swollen thyroid)
• You take a medicine that can slow the thyroid, such as amiodarone (a heart rhythm medicine) or lithium
• You have symptoms with no clear cause, such as low mood, poor memory, anaemia (low red blood cells), constipation, weight gain, weak muscles or high cholesterol

Symptoms of menopause can look very much like a slow thyroid, which is another reason a blood test helps. We do not test while you are acutely unwell, because a short illness can change the results for a while.⁶

Common signs of an underactive thyroid include:³

• Feeling tired
• Feeling cold when others are comfortable
• Dry skin, and coarse hair or hair loss
• Weight gain
• Swelling (puffiness)
• Muscle cramps
• A hoarse voice
• Poor memory or low mood
• Heavy or irregular periods, or difficulty getting pregnant

Physicians who focus on optimal hormone levels look for a typical pattern: symptoms are worst in the morning and at rest, and get better once you start moving. They also ask about:

• Trouble getting up in the morning, and feeling more tired at rest than when active
• Cold hands and feet
• A puffy face and swollen eyelids in the morning
• Stiff joints in the morning
• Constipation
• Feeling as if you are living in slow motion

These signs come on slowly, and people with a healthy thyroid can have them too.¹³ Only a blood test can show whether your thyroid is the reason.

We don’t ask you to take our word for it. This is what research has found when people with a high TSH (a thyroid running low) were treated:

• Cholesterol. In a combined analysis of 5 placebo-controlled trials (253 adults), treatment lowered “bad” LDL cholesterol by 14%, while it rose by 2% in the placebo group.⁷
• Blood vessels. A combined analysis of 3 randomised trials (117 adults) found that the wall of the neck artery thickened less with treatment, and the effect was clearer after more than 6 months. A thicker artery wall is an early sign of hardening of the arteries.⁸
• Heart. A study that pooled data from 55,287 people found that people with a clearly high TSH who were not treated had more heart disease. The researchers noted that this kind of study cannot show whether treatment lowers that risk.⁹
• Tiredness. In a study of 92 people, most of whom had a clearly underactive thyroid, the share who felt tired fell from 46% to 26% after 6 months of treatment. This study had no comparison group, so it is early research.¹⁰

Where research has not shown a benefit. A combined analysis of 21 trials (2,192 adults) with a mildly high TSH found that treatment did not improve overall quality of life or thyroid symptoms.¹¹ A trial of 737 adults aged 65 and over found the same for thyroid symptoms and tiredness.¹² The researchers noted that few of their participants had strong symptoms, and that aiming for a different TSH level might give different results.^11,12

So far, no study has looked at people whose results are in the normal range but not at the optimal level, which is the group we treat. We tell you this openly before you decide.

We look after people whose thyroid is not low enough to count as a disease, but is not at the optimal level either.

• If your results show a thyroid disease, we refer you to a hospital endocrinologist (a hormone specialist). Thyroid disease may need several specialists working together, and a hospital is set up to do this.
• If your results are in the normal range but not at the optimal level, and you have symptoms, our doctor may consider bringing your thyroid hormone to the optimal level, for your long-term health.

Standard Thai and international thyroid guidelines do not recommend treatment when results are in the normal range.^3,13 This makes our treatment off-label (used outside the medicine’s official approval). The goal is to ease your symptoms. Choosing not to take it, or asking to see an endocrinologist, is always a reasonable choice.

• Why not too low. A thyroid that stays low can raise your cholesterol and affect your heart and blood vessels over time.¹³
• Why not too high. More is not better. Too much thyroid hormone raises the risk of an irregular heartbeat called atrial fibrillation, especially as we get older, and speeds up bone loss, especially after menopause.^4,13

We use standard medical guidelines to set our safety limits. Within those limits, we aim for the optimal range, where you are most likely to benefit.

• TSH, free T4 and free T3 (together called TFT). TSH is the most reliable sign of how your thyroid is working, so your doctor makes decisions from it. Free T4 is checked as a safety limit. Free T3 is recorded in your file.^2,3
• Anti-TPO, once. This antibody test shows whether your immune system is attacking your thyroid (autoimmune thyroiditis). If it is high, you follow a different plan, which your doctor will explain.³
• A second test 3 months later, before you start. TSH can change for a while for many reasons, so we confirm your result before you take any medicine.³
• In the morning. TSH goes up and down during the day, so we test at the same time of day each time.³
• Stop biotin for 3 days before your blood test. Biotin is often found in hair, skin and nail vitamins and in multivitamins. It can make thyroid results look wrong, and can even make them look like an overactive thyroid.^3,4

Your thyroid tests are included in your Annual Panel.

Thyroid medicine is not like a vitamin. The dose that helps and the dose that is too much are very close together.⁴

• Different makers are not the same. Two products with the same name and strength can give different amounts in your blood, so they cannot be swapped without a new blood test.^4,14
• It is not a weight-loss medicine. In people with a normal thyroid, it does not help weight loss, and large doses can be dangerous, especially with appetite-suppressing medicines.⁴
• Our doctor does not give it to everyone. Because the safe range is narrow, our doctor also makes sure you understand how to take it and can come back for your blood tests. This protects you from taking it the wrong way.

Some people need special consideration before they start, because the medicine could affect another condition, or because a specialist already looks after them. These people need closer care. Before you start, our doctor checks whether any of these apply to you:

• Heart disease, an irregular heartbeat, or high blood pressure that is not yet treated
• Being 50 or older, or having several health conditions
• A lump or swelling in your thyroid
• Thyroid disease already treated by another doctor, or past thyroid surgery or radioactive iodine
• Problems with your adrenal or pituitary glands
• Pregnancy, or planning a pregnancy
• Thin bones (osteoporosis), diabetes, or a blood thinner such as warfarin

For thin bones, diabetes or a blood thinner, we can start, and we follow you more closely. For heart conditions, untreated high blood pressure, or age 50 and over, we do not start, because the risk to your heart would be greater than the benefit. For the other conditions, a specialist doctor who looks after that condition knows your situation best, so we refer you to them rather than start treatment ourselves.^3,4

We start treatment only in adults aged 40 to 49. If you are younger and have symptoms, we advise a check-up with a hospital doctor.

Your doctor sets your dose from your blood test.

• We always start low and change the dose in small steps, because the safe range is narrow.⁴
• We use one product from the same maker every time, so your blood level stays steady.¹⁴
• You take it once a day at bedtime, on an empty stomach, so it is absorbed the same way every day.¹³
• We have an upper limit. If you would need more than that, an endocrinologist is the right doctor to look after you.
• Treatment is for members. You see our doctor at every follow-up blood test, at the clinic or by video call, and you collect your medicine at the clinic. We do not deliver medicines.

We also check how you feel. If your thyroid has been on target for 6 months and your symptoms have not improved, you can choose to continue or to stop.

Follow-up blood tests are an essential part of this treatment. Almost half of people who take thyroid medicine around the world end up with too little or too much.¹⁴ Your blood tests are how we keep you in the safe range.

• 6 weeks after you start, and after every dose change. The medicine stays in your body for about a week, so your level takes about 6 weeks to settle. Testing sooner would show a number that is still changing, and could lead to the wrong dose.^4,13
• Then at 3 months, and every 6 months. Once your dose is right, regular checks keep it that way. We also ask about your symptoms at each visit.
• A short break once a year, for some people. If you take the highest dose we use and your level is still not on target, we pause the medicine for 6 weeks once a year. This shows whether your own thyroid has become weaker underneath the medicine. If it has, an endocrinologist should look after you.

If you cannot come back for the 6-week blood test, we recommend that you do not start.

1. Rousset B, Dupuy C, Miot F, Dumont J. Thyroid Hormone Synthesis and Secretion. In: Feingold KR, et al., editors. Endotext. MDText.com; updated 2015.
2. Sinha R, Yen PM. Cellular Action of Thyroid Hormone. In: Feingold KR, et al., editors. Endotext. MDText.com; updated 2018.
3. The Endocrine Society of Thailand. Recommendations for Thyroid Disorders 2025 (B.E. 2568) (in Thai). Bangkok: The Endocrine Society of Thailand; 2025.
4. EMD Serono (Merck KGaA). Levothyroxine sodium tablets — Product Monograph including Patient Medication Information. Canada.
5. Kim MI, Bertot BE. Hypothyroidism in Older Adults. In: Feingold KR, et al., editors. Endotext. MDText.com; updated 2025.
6. National Institute for Health and Care Excellence (NICE). Thyroid disease: assessment and management (NG145). Published 2019, updated 2023.
7. Abreu IM, Lau E, de Sousa Pinto B, Carvalho D. Subclinical hypothyroidism: to treat or not to treat, that is the question! A systematic review with meta-analysis on lipid profile. Endocr Connect. 2017;6:188–199. doi:10.1530/EC-17-0028
8. Zhao T, Chen B, Zhou Y, et al. Effect of levothyroxine on the progression of carotid intima-media thickness in subclinical hypothyroidism patients: a meta-analysis. BMJ Open. 2017;7:e016053. doi:10.1136/bmjopen-2017-016053
9. Rodondi N, den Elzen WPJ, Bauer DC, et al. Subclinical Hypothyroidism and the Risk of Coronary Heart Disease and Mortality. JAMA. 2010;304(12):1365–1374. doi:10.1001/jama.2010.1361
10. Ruíz-Pacheco MG, Hernández I, Hernández-Estrella G, et al. Severity of Fatigue and Its Relationship with TSH before and after Levothyroxine Replacement Therapy in Patients with Primary Hypothyroidism. Biomedicines. 2023;11:811. doi:10.3390/biomedicines11030811
11. Feller M, et al. Association of thyroid hormone therapy with quality of life and thyroid-related symptoms in patients with subclinical hypothyroidism: a systematic review and meta-analysis. JAMA. 2018;320(13):1349–1359. doi:10.1001/jama.2018.13770
12. 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. doi:10.1056/NEJMoa1603825
13. Jonklaas J, Bianco AC, Bauer AJ, et al. Guidelines for the Treatment of Hypothyroidism: Prepared by the American Thyroid Association Task Force on Thyroid Hormone Replacement. Thyroid. 2014;24(12). doi:10.1089/thy.2014.0028
14. Centanni M, Duntas L, Feldt-Rasmussen U, et al. ETA guidelines for the use of levothyroxine sodium preparations in monotherapy to optimize the treatment of hypothyroidism. Eur Thyroid J. 2025;14:e250123. doi:10.1530/ETJ-25-0123