Your thyroid is a small, butterfly-shaped gland at the front of your neck, but it influences almost every system in your body. Thyroid hormones control how quickly your body uses energy, affecting your heart rate, weight, temperature, mood and more. When the gland makes too little or too much hormone, the effects can be wide-ranging and easy to mistake for other problems.
Underactive vs overactive at a glance
| Hypothyroidism (underactive) | Hyperthyroidism (overactive) | |
|---|---|---|
| Energy | Tiredness, sluggishness | Restlessness, nervousness, poor sleep |
| Weight | Weight gain | Weight loss despite good appetite |
| Temperature | Feeling cold | Feeling hot, sweating |
| Heart | Slow heart rate | Fast or irregular heartbeat, palpitations |
| Skin and hair | Dry skin, coarse or thinning hair | Warm, moist skin, fine hair |
| Bowels | Constipation | Frequent bowel movements |
| Mood | Low mood, slowed thinking | Anxiety, irritability |
| Periods | Heavier or irregular | Lighter or less frequent |
| Other | Puffy face, hoarse voice, muscle aches | Tremor, muscle weakness, eye changes |
Hypothyroidism
Hypothyroidism is common, particularly in women and people over 60. The most frequent cause in the United States is Hashimoto’s thyroiditis, an autoimmune condition in which the immune system gradually damages the thyroid. Other causes include previous thyroid surgery or radioactive iodine, radiation to the neck, some medications such as lithium and amiodarone, and, worldwide, iodine deficiency.
Symptoms develop slowly over months or years, so many people put them down to age or stress.
Hyperthyroidism
The most common cause is Graves’ disease, an autoimmune condition in which antibodies stimulate the thyroid to overproduce hormone. Graves’ disease can also affect the eyes, causing bulging, gritty or irritated eyes and, less commonly, double vision. Other causes include thyroid nodules that produce excess hormone, inflammation of the thyroid (thyroiditis), and taking too much thyroid medication.
Untreated hyperthyroidism can lead to atrial fibrillation, heart failure and bone loss, so it should not be ignored.
The blood tests
TSH (thyroid-stimulating hormone) is usually the first test. TSH comes from the pituitary gland and tells the thyroid how much hormone to make. It works like a thermostat, so the results move in the opposite direction to thyroid activity:
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| Result | Usually means |
|---|---|
| High TSH, low free T4 | Hypothyroidism |
| High TSH, normal free T4 | Subclinical (mild) hypothyroidism |
| Low TSH, high free T4 and/or T3 | Hyperthyroidism |
| Low TSH, normal free T4 and T3 | Subclinical hyperthyroidism |
A typical TSH reference range is roughly 0.4 to 4.0 mIU/L, though labs vary. Antibody tests (TPO antibodies for Hashimoto’s, TSH receptor antibodies for Graves’) help identify the cause. An ultrasound or a thyroid uptake scan may be used to look at nodules or the cause of an overactive thyroid.
Biotin supplements, often found in hair and nail products, can interfere with some thyroid blood tests. Stop them for a few days before testing if your doctor advises.
Treating hypothyroidism
The standard treatment is levothyroxine, a synthetic form of the T4 hormone, taken once a day. Tips for taking it:
- Take it on an empty stomach, usually first thing in the morning, 30 to 60 minutes before breakfast or coffee
- Keep calcium, iron supplements and antacids at least four hours apart from your dose
- Take it consistently, at the same time and in the same way each day
Your doctor will check TSH six to eight weeks after starting or changing the dose, then usually once a year when stable. Most people need treatment for life, but once the dose is right, symptoms usually improve within a few weeks to months.
Treating hyperthyroidism
There are three main approaches:
- Antithyroid medicines, usually methimazole, which reduce hormone production. Some people with Graves’ disease go into remission after 12 to 18 months of treatment.
- Radioactive iodine, taken as a capsule or drink, which shrinks the overactive thyroid. Many people then need levothyroxine for life.
- Surgery to remove all or part of the thyroid, often used for large goiters, suspicious nodules, or when other treatments are not suitable.
Beta blockers such as propranolol are often used at first to control a fast heart rate, tremor and anxiety while other treatment takes effect.
Thyroid and pregnancy
Thyroid hormone is vital for a baby’s brain development. Women with known thyroid disease should talk to their doctor before trying to conceive, because levothyroxine doses usually need to increase early in pregnancy. Some antithyroid medicines are preferred over others during different stages of pregnancy.
Thyroid nodules
Lumps in the thyroid are very common and most are harmless. Your doctor may arrange an ultrasound and, if a nodule has certain features, a fine-needle biopsy to rule out cancer.
When to see a doctor
See a doctor if you notice a combination of the symptoms above, a swelling in your neck, or eye changes. Get urgent help for a very fast or irregular heartbeat, chest pain, confusion or a high fever with signs of an overactive thyroid.
The bottom line
An underactive thyroid slows the body down and an overactive one speeds it up. Both are common, easy to test for with a simple blood test, and very treatable. If you feel persistently “not yourself,” ask your doctor whether a thyroid check makes sense.
This article is for general information and does not replace medical advice.