Download FULL Free Review of Thyroid Disease and Metabolism
By Dr. Richard Lipman, MD
Board-Certified Internal Medicine/endocrinology
Reviewed and updated August 15, 2026
Do You Have a Low Thyroid Causing Problems Losing Weight?
Low thyroid weight gain is a common problem, especially in women. Often termed ” underactive thyroid” or hypothyroidism, it is usually the cause of unexpected weight gain. The greater the degree of hypothyroidism (low thyroid function), the greater is the weight gain. This is all because the thyroid gland regulates our basal metabolism, as well as excess fat accumulation, sodium, and water retention. As an endocrinologist, Dr. Lipman has treated thousands of individuals with both hyper- and hypothyroidism. Many are complicated by extreme weight gain. Any GLP-1 diet program starts with a thyroid check—a simple TSH test can cost $10!
Up to 300 million people worldwide suffer from an underactive thyroid or hypothyroidism. In the US, 4.6% of the population (about 5 out of every 100 people) suffers from low thyroid. Mounting scientific research links low thyroid function to stubborn weight loss resistance and countless other serious health conditions.
What Function Does the Thyroid Gland Play in Metabolism?
The thyroid gland is a butterfly-shaped gland located in the center of the neck. Your thyroid produces two vital metabolism-regulating hormones: triiodothyronine (T3) and thyroxine (T4). Your pituitary gland controls both through the thyroid-stimulating hormone (TSH).
The function of the thyroid gland is to take iodine, found in many foods, and convert it into thyroid hormones: thyroxine (T4) and triiodothyronine (T3). Thyroid cells are the only cells in the body that can absorb iodine. These cells combine iodine and the amino acid tyrosine to make T3 and T4. T3 and T4 are then released into the bloodstream and transported throughout the body, where they regulate metabolism (the conversion of oxygen and calories into energy).
HYPOTHYROIDISM IN WOMEN: Low Thyroid Function and Weight Gain
Ten times as many women as men have an underactive thyroid. It typically starts between the ages of 40 and 50, along with the other hormonal changes occurring at this stage in a woman’s life. More and more women start experiencing the three most common hypothyroid symptoms (weight gain, fatigue/depression, and skin changes) as they reach these ages. The most common problem is differentiating these symptoms, which are shared by many other diseases, from the true hypothyroid state. When all three of the symptoms occur together, it’s easy. However, more often than not, only 1 or 2 symptoms may be present at a time. For borderline hypothyroid cases, often none of the symptoms are present except for the inability to lose weight.
SIGNS OF LOW THYROID
Note the significant overlap with other medical conditions, which often makes the diagnosis difficult. Unfortunately, many of these signs and symptoms overlap with serious as well as more common symptoms such as anxiety, depression, muscle cramps, and swelling. For this reason, only a thyroid blood test can tell you your thyroid status.
What blood tests are used to Indicate Thyroid Status?
Thyroid weight gain is typically evaluated by the TSH test. TSH is the key hormone for diagnosing hyperthyroidism and hypothyroidism. If the results of the TSH test are abnormal, one or more additional tests are needed to help determine the cause of the problem.
Thyroid Weight Gain and TSH Test
This blood test is the most sensitive test of thyroid function available. The TSH test can detect TSH blood levels as low as 0.01 mIU/L. The normal range for TSH is between 0.3 and 4.5 mIU/L, although the range may vary slightly from one laboratory to another. Between 3 and 4.0 is borderline, and 4.5 or higher indicates low thyroid function. The higher the TSH, the lower the thyroid!
The TSH test measures how TSH and thyroid hormones interact. Normally, the pituitary increases TSH secretion when blood thyroid hormone levels are low. The thyroid responds by making more hormone. Then, when the body has enough thyroid hormone circulating in the blood, TSH output drops. The cycle repeats continuously to maintain healthy thyroid hormone levels in the body. The TSH test measures the amount of TSH secreted by the pituitary gland. In people whose thyroid produces too much thyroid hormone, the pituitary shuts down TSH production, leading to low or even undetectable TSH levels in the blood. An abnormally low TSH level suggests too much thyroid hormone (high T4 and T3), hyperthyroidism. In people whose thyroid gland does not function normally and produces too little thyroid hormone, the thyroid gland cannot respond normally to TSH by producing thyroid hormone. As a result, the pituitary continues to produce TSH, trying to stimulate the thyroid to respond.
An abnormally high TSH level( above 4.5) suggests low thyroid function- hypothyroidism and possible weight gain.
T4 Tests in Low and High Thyroid Conditions
T4 is the principal thyroid hormone. The normal range for total T4 is 4.5 to 12.6 (µg/dL), although again, the range may vary slightly from one laboratory to another. Elevated total T4 suggests hyperthyroidism, and low total T4 points to hypothyroidism. Some conditions such as pregnancy or the use of oral contraceptives alter the levels of T4 and T3. In those cases, one should use only the TSH test. Steroids and chronic disease may actually lower the T4; again, use the TSH.
T3 is not a good test for Thyroid Weight Gain
Only about 20 percent of the T3 circulating in the blood comes from the thyroid gland, while all of the circulating T4 comes from the thyroid. The remaining 80 percent of circulating T3 comes from various cells throughout the body, where T4 is converted to T3. T3 is far more active than T4 . The normal FT3 range is about 0.2 to 0.5 ng/dL. The T3 test is not useful in diagnosing hypothyroidism because levels are not reduced until the hypothyroidism is severe.
Imaging Tests in Thyroid Disease
A physician may use one or more imaging tests, such as a thyroid ultrasound, a computed tomography (CT) scan, or nuclear medicine tests, to diagnose and identify the cause of thyroid disorders. Thyroid imaging is reviewed in the National Institutes of Health report on thyroid disease.
What to Do If You Suspect Low Thyroid Function Causing Your Weight Gain?
Reviewing the table of symptoms above, you will see that many symptoms suggest a low thyroid state. Many overlap with other diseases. Since thyroid disease is often genetic, the first step is reviewing your family history. More common in females than males, any woman with unexplained weight gain and a family history of hypothyroidism needs a TSH test. This is simple and definitive. If your symptoms include weight gain, fatigue, hair loss, and dry skin, you should obtain a TSH test. If you have difficulty losing weight on a low-carb/low-calorie diet, you also need a TSH test. If you are a “slow responder” on GLP-1 you still need a TSH. Treatment of hypothyroidism involves simply taking thyroid hormone in the form of Cytomel and/or Synthroid. Once your thyroid status is normalized, you can start your GLP-1’s, if you have not lost the weight by simply taking the thyroid pills!
