Thyroid dysfunction is often called a "great imitator" because its symptoms, such as fatigue, weight change, anxiety, low mood, palpitations and brain fog, closely mimic other conditions. An underactive or overactive thyroid can look like depression, an anxiety disorder, heart disease, menopause or simply "getting older," which is why testing, not guessing, is essential.
As an internist, I check thyroid function often, because missing it means treating the wrong problem.
Why Thyroid Dysfunction Is So Easy to Miss
The thyroid is a small gland in the neck that sets the pace of metabolism in nearly every tissue. When it runs too slow (hypothyroidism) or too fast (hyperthyroidism), the effects show up everywhere: brain, heart, gut, skin, muscles, reproductive system and cholesterol levels.
That body-wide reach is exactly what makes it a great imitator. Symptoms develop gradually, they are nonspecific, and patients often see different specialists for different pieces of the picture.
Hypothyroidism: When Everything Slows Down
Common symptoms and findings include:
- Fatigue and low energy
- Weight gain or difficulty losing weight
- Feeling cold
- Constipation
- Dry skin, brittle hair and hair thinning
- Low mood, slowed thinking and poor concentration
- Heavy or irregular menstrual periods, and fertility problems
- Muscle aches and cramps
- Elevated LDL cholesterol
- Slow heart rate
What it imitates: depression, chronic fatigue, perimenopause, weight-loss resistance, high cholesterol that seems "genetic," and normal aging.
In the U.S., the most common cause is Hashimoto's thyroiditis, an autoimmune condition. Other causes include prior thyroid surgery or radioactive iodine, and medications such as lithium and amiodarone.
Hyperthyroidism: When Everything Speeds Up
Common symptoms and findings include:
- Anxiety, irritability and restlessness
- Palpitations or a racing heart
- New atrial fibrillation
- Unintentional weight loss despite a normal or increased appetite
- Tremor
- Heat intolerance and sweating
- Insomnia
- Frequent bowel movements
- Muscle weakness and bone loss over time
What it imitates: panic disorder, generalized anxiety, primary heart rhythm problems, eating disorders and stress.
Causes include Graves' disease, toxic nodules, and inflammation of the thyroid (thyroiditis), which can follow pregnancy or a viral illness.
How Thyroid Dysfunction Is Diagnosed
Testing is straightforward, but interpreting it well takes care:
- TSH (thyroid-stimulating hormone): the best single screening test for most people. A high TSH usually suggests an underactive thyroid; a low TSH suggests an overactive one.
- Free T4: helps confirm and grade abnormal TSH results.
- T3: most useful when hyperthyroidism is suspected.
- Thyroid antibodies (TPO, and TSH receptor antibodies when indicated): identify autoimmune causes such as Hashimoto's or Graves' disease.
- Thyroid ultrasound: when there is a nodule or enlargement on exam.
Common pitfalls I watch for
- Biotin supplements can interfere with some lab assays and produce misleading results. I ask patients to pause biotin before testing.
- Acute illness can temporarily shift thyroid numbers ("non-thyroidal illness"); it is often wise to repeat tests after recovery.
- Pituitary problems can produce a misleading TSH.
- Pregnancy changes normal ranges, and thyroid status matters for the baby.
- A single mildly abnormal result often deserves repeat testing in several weeks before a lifelong diagnosis.
Subclinical thyroid disease
Sometimes TSH is slightly abnormal while free T4 is normal. Whether to treat depends on the degree of abnormality, symptoms, age, antibodies, pregnancy plans and heart health. This is an area where individualized judgment matters more than a single number.
Who Should Be Tested
Routine thyroid screening for people without symptoms is debated, but testing is clearly reasonable when there are symptoms or risk factors. I have a low threshold to check thyroid function in people who have:
- Unexplained fatigue, weight change, mood change or palpitations
- New atrial fibrillation, high cholesterol or unexplained anemia
- A personal or family history of thyroid disease or other autoimmune conditions, such as type 1 diabetes, celiac disease or vitiligo
- Pregnancy, plans for pregnancy, or a recent delivery
- Prior radiation to the head or neck
- Medications known to affect the thyroid, including lithium, amiodarone and certain cancer immunotherapies
Women are affected considerably more often than men, and the risk of thyroid disease rises with age, so a symptom that seemed minor at 35 deserves a fresh look at 55.
Treatment Basics
- Hypothyroidism is usually treated with levothyroxine, an FDA-approved medication with labeled dosing that is adjusted based on follow-up labs and symptoms.
- Hyperthyroidism may be treated with anti-thyroid medication, radioactive iodine or surgery, and beta-blockers can control symptoms in the meantime. These decisions often involve an endocrinologist.
Beyond the Thyroid: When Labs Are Normal
If thyroid tests are normal and symptoms persist, the work is not done. Fatigue, weight gain and brain fog can come from iron deficiency, vitamin B12 deficiency, sleep apnea, depression, insulin resistance, low testosterone, perimenopause, medication side effects and more. A broad baseline panel is the subject of lab testing before longevity or peptide discussions.
How I Approach Thyroid Concerns at Laeeq M.D.
At Laeeq M.D., a thyroid evaluation is part of a complete internal medicine assessment: a detailed symptom history, exam, medication and supplement review, and targeted labs interpreted in context. For patients already diagnosed, I focus on careful dose adjustment and long-term monitoring. For a longevity-oriented look at thyroid health, see thyroid optimization for longevity.
Book a Consultation
If you have symptoms that have not been explained, or you want a careful second look at your thyroid labs, a thorough evaluation is the right place to start. You can book a consultation for a 60-minute visit with Dr. Laeeq, virtually or in person in Reston, VA.
This article is for educational purposes and is not medical advice. Discuss any treatment with a licensed physician who knows your history.
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Written by Dr. Laeeq Ahmed Butt, M.D., MBA
Board-certified internist practicing peptide and longevity medicine in Reston, VA, with virtual care in seven states. About Dr. Laeeq