Hypothyroidism is an endocrine condition that occurs when the thyroid gland doesn’t produce and release sufficient thyroid hormone into the bloodstream. Thyroid hormone, which collectively refers to two hormones — thyroxine (T4, also known as tetraiodothyronine) and triiodothyronine (T3) — plays essential roles in metabolism, growth, and many other bodily functions. Commonly known as underactive thyroid, hypothyroidism leads to slowed metabolism, which often results in unexpected weight gain and a constant feeling of tiredness. Other common symptoms that prompt testing for hypothyroidism include cold intolerance, hoarse voice and dry skin.
The condition typically develops slowly over many years, and while symptoms may not be obvious during the early stages, undetected and untreated hypothyroidism can lead to serious health complications over time, including high cholesterol, cardiovascular problems, goiter, nerve damage, infertility and others.
How does hypothyroidism present?
Four main types of hypothyroidism are recognized, including three overt forms that require treatment and one subclinical form.
Overt hypothyroidism
Primary hypothyroidism. This is by far the most common presentation, and occurs when the thyroid is directly impacted and cannot produce sufficient thyroid hormone. Historically, iodine deficiency was a major cause of primary hypothyroidism, as iodine is a critical component in the biosynthesis of T3 and T4 thyroid hormones. Today, the chronic autoimmune condition Hashimoto’s thyroiditis is the most common cause of primary hypothyroidism. Other frequent causes include certain drugs, thyroid gland surgery or iodine-radiation therapy.
Secondary hypothyroidism. This occurs when the pituitary gland is underactive. This rare type of hypothyroidism prevents the pituitary gland from sending TSH to the thyroid, resulting in reduced stimulation of the thyroid and thus reduced production of thyroid hormone.
Tertiary hypothyroidism. This form occurs when the hypothalamus doesn’t produce enough thyrotropin-releasing hormone (TRH). As a result, the pituitary gland can’t make enough TSH, and similarly to secondary hypothyroidism, this results in reduced stimulation of the thyroid.
Subclinical hypothyroidism
Also known as mild thyroid failure, this occurs when TSH levels are slightly elevated, but all other thyroid hormone levels fall within the normal range. Subclinical hypothyroidism can and often does resolve spontaneously within a few months, but affected individuals should be monitored to eliminate the possibility of autoimmune thyroid diseases, which would increase the risk of developing overt hypothyroidism over time.
How common is hypothyroidism?
Hypothyroidism is considered a common condition whose prevalence varies according to several factors, including local dietary iodine availability, gender and age. According to a recent Lancet review, the prevalence of overt and subclinical hypothyroidism ranges from 0.2% to 5.3% in Europe and 0.3% to 3.7% in the United States, depending on the definition used and the population (1 and references within).
In general, the risk of hypothyroidism increases with age and has historically been higher in regions with poor nutrition including iodine and Vitamin D deficiency, whereby the latter is implicated in autoimmunity. While the prevalence of hypothyroidism appears to be on the rise, the apparent increase may largely reflect improved awareness and earlier diagnosis rather than a true rise in disease prevalence, particularly in regions previously burdened by iodine deficiency, e.g., India, where the prevalence was reported to be 11 % (2).
Diagnosis via thyroid stimulating hormone blood test
Because hypothyroidism symptoms are non-specific and often overlap with other conditions or the natural consequences of aging, a firm clinical diagnosis requires biochemical testing of serum thyroid function.
The most common test is the thyroid stimulating hormone blood test or TSH test. The rationale behind this is that when thyroid hormone levels become too low (as in hypothyroidism), the pituitary produces more TSH in an attempt to stimulate the thyroid. Elevated TSH levels have become the acceptable biomarker for reduced thyroid function.
During a TSH blood test, a venous blood sample is taken from the individual suspected to have hypothyroidism, and the serum levels of TSH are measured via immunoassay.
Reference ranges for thyroid function tests are traditionally based on fixed percentiles of the population distribution, but there is increasing awareness of the need for more individualized intervals based on key factors such as age, sex, and special circumstances such as pregnancy. According to the American Thyroid Association, doctors generally consider the normal range for TSH to be 0.4 to 4.0 mU/L.
However, TSH reference ranges may vary significantly depending on the ethnicity of the population tested, regional iodine status, and the specific testing method and analytical instrumentation used. The complexities of establishing appropriate TSH reference ranges are explored in detail in a recent review, which includes a nationwide cross-sectional study and meta-analysis of reference intervals (3). So, while there is much debate about reference ranges, the following values are used for non-pregnant adults by many testing labs (4):
| Hyperthyroidism | Normal | Mild hypothyroidism | Hypothyroidism |
| 0 to 0.4 mU/L | 0.4 to 4 mU/L | 4–10 mU/L | 10 mU/L |
It is recommended that if an elevated TSH concentration is found, the thyroid function test is repeated to confirm the result, because modestly elevated TSH levels can spontaneously return to the reference range without treatment in up to 50% of cases (5).
Confirmed abnormal TSH test results typically prompt further tests to measure the levels of T4 and T3 hormones as well as an antibody test to determine whether the individual has anti-thyroid antibodies, which may be an indicator of an autoimmune disorder such as Hashimoto’s or Graves’ disease.
Hypothyroidism is treatable but underdiagnosis is a major problem!
Regardless of the type or cause of hypothyroidism, the treatment approach involves thyroid hormone replacement. Today, a daily oral dose of a synthetic form of thyroxine (T4) is the standard treatment for hypothyroidism. This approach is safe and inexpensive, restores thyroid function tests to within the reference range, and relieves symptoms in the majority of patients (6 and references within).
However, underdiagnosis remains a major public health challenge. The non-specific nature of hypothyroidism symptoms, combined with their tendency to be attributed to aging or other conditions, means diagnosis often occurs late or not at all. Large observational studies and meta-analyses have shown that about 4-7% of community-derived populations in the USA and Europe have undiagnosed hypothyroidism, with approximately 80% of these cases believed to be overt and requiring treatment. The American Thyroid Association estimates that 20 million residents of the United States have some form of thyroid disease, and that as many as 60% – about 12 million people – are unaware of it (7 and references within).
While the full public health and economic burden of underdiagnosed hypothyroidism has yet to be quantified, individuals who remain undiagnosed are at higher risk of serious and potentially life-threatening complications. Low awareness, lack of knowledge about thyroid function, and the tendency to dismiss symptoms all contribute to this ongoing diagnostic challenge.
Efforts to increase awareness and more accessible testing options, including at-home, remote testing are expected to improve the diagnosis and treatment of individuals with hypothyroidism. Promising developments in this area include the upcoming launch of the first TSH testing service utilizing Capitainer samples, with validation studies demonstrating excellent correlation with traditional venous blood testing. Stay tuned for updates on this!
References:
- Taylor PN, Medici MM, Hubalewska-Dydejczyk A, Boelaert K. Hypothyroidism. Lancet. 2024 Oct 5;404(10460):1347-1364.
- Bagcchi S. Hypothyroidism in India: more to be done. Lancet Diabetes Endocrinol. 2014 Oct;2(10):778.
- What are the ranges, symptoms, and meaning of TSH levels? Medical News Today. Medically reviewed by Marina Basina, MD — Written by Jamie Eske — Updated on April 22, 2025. Accessed June 17, 2025.
- Wang X, Li Y, Zhai X, Wang H, Zhang F, Gao X, Liu S, Teng W, Shan Z. Reference Intervals for Serum Thyroid-Stimulating Hormone Based on a Recent Nationwide Cross-Sectional Study and Meta-Analysis. Front Endocrinol (Lausanne). 2021 Jun 1;12:660277.
- Meyerovitch J, Rotman-Pikielny P, Sherf M, Battat E, Levy Y, Surks MI. Serum thyrotropin measurements in the community: five-year follow-up in a large network of primary care physicians. Arch Intern Med. 2007 Jul 23;167(14):1533-8.
- Patil N, Rehman A, Anastasopoulou C, et al. Hypothyroidism. [Updated 2024 Feb 18]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2025 Jan-.
- Gottwald-Hostalek U, Schulte B. Low awareness and under-diagnosis of hypothyroidism. Curr Med Res Opin. 2022 Jan;38(1):59-64.