Hypothyroidism
Hypothyroidism, or underactive thyroid, occurs when the thyroid does not provide enough thyroid hormone for the body’s needs. Because thyroid hormones influence metabolism throughout the body, symptoms can be broad and develop gradually.
At a glance
Quick facts
- Category
- Thyroid hormone deficiency
- Common cause
- Autoimmune thyroid disease
- Typical test
- TSH ± free T4
- First-line treatment
- Levothyroxine
- Symptoms
- Often gradual and nonspecific
- Monitoring
- TSH-based in primary hypothyroidism
Dr. Augustine Chinemerem Anadu
Consultant Family Physician · University of Calabar Teaching Hospital
Reviewed for clinical accuracy, practical wording and general-medicine context.
Names & terminology
How this entity may be named or searched
- Hypothyroidism
- Insufficient thyroid hormone effect in the body.
- Primary hypothyroidism
- The thyroid gland itself is underactive; this is the common pattern.
- Subclinical hypothyroidism
- TSH is elevated while circulating free T4 remains within the reference range.
Structured entity profile
Clinical profile
Overview
Hypothyroidism develops when the thyroid does not make enough hormone. The most common causes include autoimmune thyroid disease, previous thyroid surgery or radioiodine treatment, some medicines and less common pituitary or hypothalamic disease. [1] [2]
Symptoms alone cannot establish the diagnosis because fatigue, weight change, mood symptoms and other complaints are nonspecific. Blood testing is required. [1] [2]
Symptoms and warning signs
Symptoms can include fatigue, feeling unusually cold, weight gain, constipation, dry skin, muscle aches, difficulty concentrating, poor memory or low mood, menstrual changes and a slower heart rate. The pattern varies widely and symptoms can be mild, especially early in the condition. [2]
SYMPTOM MATRIX
Hypothyroidism can affect many body systems
Symptoms are often gradual and overlap with common non-thyroid conditions.
- Fatigue or low energy
- Cold intolerance
- Constipation
- Dry skin
- Weight gain or difficulty losing weight
- Slower thinking or low mood
- Muscle aches or weakness
- Menstrual changes
- Hair thinning
- Slow heart rate
- Marked drowsiness or confusion
- Hypothermia
- Very slow heart rate with severe illness
- Breathing or circulatory compromise
Severe decompensated hypothyroidism is rare but can be life-threatening and requires emergency care.
Diagnosis
For most adults in whom pituitary disease is not suspected, NICE recommends measuring TSH first. If TSH is above the laboratory reference range, free T4 is measured in the same sample. When pituitary disease is suspected, TSH and free T4 should be measured together. [1]
Thyroid peroxidase antibodies can support an autoimmune cause in adults with elevated TSH, but repeated antibody testing is generally not needed. High-dose biotin supplements can interfere with some thyroid assays and should be discussed before testing. [1]
Diagnostic pathway
Confirm thyroid dysfunction with laboratory testing
The pattern of TSH and free T4 helps distinguish primary hypothyroidism from other causes.
TSH and thyroid-hormone reference ranges vary by laboratory and clinical context, including pregnancy. Source [1]
Common causes
Autoimmune Hashimoto disease is a common cause. Hypothyroidism can also follow thyroid surgery, radioiodine treatment or neck irradiation, and can be caused by some medicines or iodine imbalance. Pituitary disease produces a different hormone pattern and requires specialist interpretation. [2]
Treatment and management
NICE recommends levothyroxine as first-line treatment for primary hypothyroidism. The dose is individualized according to age, body size, cardiovascular disease, pregnancy and the degree of hormone deficiency. [1]
NICE does not recommend routine liothyronine alone or in combination with levothyroxine for primary hypothyroidism, and does not recommend natural thyroid extract, because evidence of additional benefit is insufficient and long-term harms are uncertain. [1]
Treatment & care hub
Replace thyroid hormone and monitor for the right dose
Treatment aims to restore thyroid function without under- or over-replacement.
Replace missing thyroid hormone
Levothyroxine is the standard first-line treatment for primary hypothyroidism.
Follow TSH over time
TSH is checked after dose changes and periodically once stable.
Review timing and medicines
Some foods, supplements and medicines can reduce levothyroxine absorption or alter thyroid tests.
Review promptly
Thyroid requirements and monitoring change during pregnancy and preconception care.
Follow-up and monitoring
NICE advises checking TSH more frequently while treatment is being stabilized and then typically annually in stable adults. Free T4 can also be checked when symptoms persist despite levothyroxine treatment. [1]
United States estimate
Hypothyroidism is common, and many cases are mild
NIDDK reports that nearly 5 in 100 Americans aged 12 years and older have hypothyroidism.
Common questions
Frequently asked questions
TSH is usually the first blood test in adults when pituitary disease is not suspected; free T4 is added when needed.
No. Symptoms are nonspecific and diagnosis requires thyroid blood tests.
Levothyroxine is first-line treatment for primary hypothyroidism.
It depends on the cause. Autoimmune or post-surgical hypothyroidism is often permanent, while some thyroiditis-related cases can be temporary.
Yes. High-dose biotin can interfere with some thyroid laboratory assays, so supplement use should be discussed before testing.
Sources
- NICE — Thyroid disease: assessment and management (NG145)
- NIDDK — Hypothyroidism (Underactive Thyroid)
This article is based on trusted medical sources and has undergone clinician review. It is not a substitute for personalized medical advice.
Patients often describe difficulty concentrating or poor memory more clearly than “slowed thinking.” Hypothyroidism is also substantially more common in women.