An underactive thyroid, medically called hypothyroidism, occurs when the thyroid gland doesn’t produce enough thyroid hormone, causing nearly every system in the body to slow down. Common signs include persistent fatigue, unexplained weight gain, cold intolerance, dry skin, constipation, and low mood. The condition affects around 5 people in 100 and develops slowly, often over years, which means symptoms frequently go unnoticed or get attributed to ageing, stress, or other conditions.
What Is an Underactive Thyroid?
The thyroid is a small, butterfly-shaped gland in the front of the neck that produces hormones controlling metabolism, the rate at which your body converts food into energy. When the thyroid becomes underactive, hormone levels drop, and body processes slow down: heart rate, digestion, temperature regulation, and cognitive processing all become sluggish. Hashimoto’s disease, an autoimmune condition where the immune system attacks the thyroid gland, is the most common cause in the UK and US, typically starting between ages 30 and 50 and affecting women considerably more often than men.
Physical Signs
- Persistent fatigue: often described as bone-deep exhaustion that doesn’t improve with rest; usually the first and most reported symptom
- Unexplained weight gain: typically 5-15 pounds, driven by metabolic slowdown and fluid retention rather than increased fat mass
- Cold intolerance: feeling cold when others are comfortable, since reduced thyroid hormone lowers the body’s heat production (thermogenesis)
- Constipation: slowed gut motility is a classic but frequently overlooked sign
- Dry skin and brittle nails: reduced sweating and slower skin cell turnover
- Hair thinning: hair may thin diffusely across the scalp, and the outer third of the eyebrows thinning or disappearing is a classic, specific sign
- Puffy face and swelling: caused by a buildup of mucopolysaccharides in tissues, a condition called myxedema in more severe cases
- Slow heart rate: bradycardia occurs in an estimated 12-15% of patients
- Muscle aches and joint pain: generalised weakness, cramps and stiffness, particularly in the shoulders and hips
- Hoarse voice
Cognitive and Mood Signs
- Brain fog: difficulty concentrating, forgetfulness, and mental sluggishness, sometimes mistaken for early cognitive decline in older adults
- Depression and low mood: thyroid hormones regulate mood-related neurotransmitters directly; studies suggest 10-15% of people referred for depression actually have undiagnosed thyroid dysfunction
- Anxiety: less commonly recognised, but documented cases show hypothyroidism symptoms closely resembling generalised anxiety disorder, sometimes leading to misdiagnosis
If low mood or anxiety symptoms have persisted without an obvious cause, our guide to signs of emotional exhaustion covers another common source of persistent fatigue and low mood worth ruling out alongside a thyroid check, since the two can present very similarly.
Other Signs
- Heavy, frequent, or irregular menstrual periods, and potential fertility difficulties
- Elevated cholesterol, sometimes the only detectable sign in older adults, since hypothyroidism impairs cholesterol clearance from the blood
- Numbness or tingling in the hands
Signs of an Underactive Thyroid in Older Adults
| Sign | Why It’s Different in Older Adults |
|---|---|
| Unexplained high cholesterol | May be the only detectable sign, warranting a thyroid check on its own |
| Heart failure symptoms | Reduced blood volume and weaker heart contractions can contribute to heart failure |
| Bowel changes | Constipation is more common; less often, diarrhoea occurs, which is more typical of an overactive thyroid |
| Vague joint or muscle pain | Sometimes the only presenting symptom in older patients |
Symptoms in people over 60 are frequently more confusing and easily attributed to other age-related conditions, which is one reason hypothyroidism often goes undiagnosed in this age group for extended periods.
What Causes an Underactive Thyroid?
- Hashimoto’s disease: an autoimmune condition and the most common cause, where the immune system gradually attacks and damages thyroid tissue
- Thyroid surgery or radiation treatment: for thyroid cancer or an overactive thyroid, which can remove or damage the gland’s hormone-producing capacity
- Iodine deficiency: the body needs iodine to produce thyroid hormone; this is a leading global cause, though rarer where salt iodisation is standard
- Certain medications: including lithium and amiodarone
- Postpartum thyroiditis: temporary thyroid inflammation following pregnancy
- Pituitary gland problems: since the pituitary triggers thyroid hormone release, dysfunction here can cause secondary hypothyroidism even with a healthy thyroid gland
How Hypothyroidism Is Diagnosed
Diagnosis relies on a blood test measuring thyroid-stimulating hormone (TSH), the pituitary hormone that signals the thyroid to produce more hormone. Elevated TSH indicates the thyroid isn’t producing enough hormone on its own, since the pituitary compensates by increasing its signal. Doctors also measure thyroxine (T4) directly, and sometimes antibody tests to confirm an autoimmune cause like Hashimoto’s disease. T3 testing isn’t routine, since levels often remain normal even with significantly low thyroid function.
Underactive vs Overactive Thyroid
| Feature | Underactive (Hypothyroidism) | Overactive (Hyperthyroidism) |
|---|---|---|
| Metabolism | Slows down | Speeds up |
| Weight | Gain | Loss |
| Heart rate | Slow | Fast, palpitations |
| Temperature tolerance | Cold intolerance | Heat intolerance |
| Bowel habits | Constipation | Diarrhoea |
| Hair | Thinning, brittle | Thinning, fine |
How Is an Underactive Thyroid Treated?
Treatment involves daily levothyroxine tablets, a synthetic version of the thyroxine hormone the thyroid isn’t producing enough of. Most people need this medication for life, with regular blood tests initially to find the correct dose. Getting the dose right can take some trial and adjustment; too much or too little levothyroxine can cause side effects including nausea, headaches, diarrhoea, or sleep problems. Once correctly dosed, most people manage the condition well and live a normal life without significant symptoms.
What Happens if an Underactive Thyroid Goes Untreated?
Untreated hypothyroidism can lead to infertility, obesity, heart problems, peripheral neuropathy, and an enlarged thyroid gland (goitre). In very rare, severe cases, it can progress to myxoedema coma, a life-threatening emergency requiring hospital treatment, though this is uncommon with modern diagnosis and treatment access. During pregnancy, untreated hypothyroidism specifically increases the risk of complications including pre-eclampsia, birth defects, and miscarriage, which is why thyroid function is closely monitored if you’re pregnant or planning pregnancy.
What Are the Earliest Signs of an Underactive Thyroid?
The earliest signs of an underactive thyroid are typically persistent fatigue that doesn’t improve with rest, mild cold intolerance, and subtle weight gain despite no change in diet or exercise. Because hypothyroidism develops gradually, often over years, these early signs are commonly dismissed or attributed to stress, ageing, or lifestyle factors, which is why a simple TSH blood test is worth requesting if these symptoms persist without another clear explanation.
Frequently Asked Questions
Can an underactive thyroid cause depression?
Yes. Thyroid hormones directly influence neurotransmitters involved in mood regulation. Some research suggests 10-15% of people referred for depression actually have an undiagnosed underactive thyroid, which is why a thyroid check is often recommended before or alongside depression treatment.
How common is an underactive thyroid?
It affects around 5 people in 100 in countries like the US and Germany, and is significantly more common in women and in people over 60, though it can develop at any age.
Can hypothyroidism go away on its own?
Sometimes, particularly if caused by temporary thyroid inflammation, such as postpartum thyroiditis, which can resolve on its own. However, hypothyroidism caused by autoimmune damage (Hashimoto’s disease) or thyroid removal is typically permanent and requires lifelong treatment.
Is weight gain from an underactive thyroid usually severe?
No. Weight gain from hypothyroidism is typically modest, around 5-15 pounds, caused mainly by metabolic slowdown and fluid retention rather than substantial fat gain. Severe or rapid weight gain usually has other or additional contributing causes.
This article is for general information only and is not a substitute for professional medical advice. If you’re experiencing symptoms of an underactive thyroid, please speak to your GP for blood testing and diagnosis.
About the Author: Written by the Well Ideas editorial team, dedicated to researching and sharing practical, evidence-informed wellness guidance for a healthier everyday life.
