What is subclinical hypothyroidism?
If you experience non-specific symptoms like mild fatigue, slight cold intolerance, or reduced ability to lose weight, and your thyroid blood test results indicate slightly elevated thyroid-stimulating hormone (TSH) with standard thyroxine (T4), you may be experiencing subclinical hypothyroidism. This state is a mild form of hypothyroidism, a condition in which the body doesn't produce enough thyroid hormones.
Subclinical hypothyroidism is defined by elevated TSH levels and T4 levels within the normal reference range. TSH is very sensitive to changes in T4. The smallest decrease in T4 can substantially increase TSH.
Symptoms of subclinical hypothyroidism
Many patients with subclinical hypothyroidism report no symptoms. Clinical symptoms of hypothyroidism present when the disease is fully developed. Still, it is not uncommon to have some more mild symptoms in the earlier stages of hypothyroidism (subclinical). There may be a correlation between higher TSH levels and the severity of symptoms.
Symptoms of hypothyroidism may include:
- Weakness, lethargy, fatigue
- Dry skin
- Coarse hair
- Cold intolerance
- Weight gain
- Muscle cramps
- Voice hoarseness
- Irregular periods
Check your symptoms with this quiz:
Causes of subclinical hypothyroidism
The cause(s) of subclinical hypothyroidism are the same that cause overt hypothyroidism. Autoimmune thyroiditis, or Hashimoto's disease, is the most common cause of hypothyroidism in the United States. The treatment of hyperthyroidism with radioactive iodine, surgery, or antithyroid medications may also cause hypothyroidism. Less common causes include the use of some drugs, including lithium and amiodarone, among others.
Progression to overt hypothyroidism
Laboratory result that indicates subclinical hypothyroidism does not necessarily mean that a patient will progress to overt hypothyroidism. Before diagnosing subclinical hypothyroidism, TSH levels should be measured again in a few months to rule out alternatives. There may have been laboratory error, recovery from a non-thyroidal illness during the time of the initial blood draw, or other antibodies that interfered with the TSH test.
In some patients, typically in those who begin with closer-to-normal TSH levels, TSH levels will return to normal within two years of the initial blood draw. Most people, however, will remain at the subclinical level with high TSH and standard T4.
Within a few years, a small percentage of patients progress to overt hypothyroidism, although for many, the condition may resolve or remain unchanged. The percentage rate goes up in those with risk factors for hypothyroidism, such as women, people over the age of 50, and those with high levels of thyroid antibodies.
Treatment for subclinical hypothyroidism
The question, then, is what do we do about subclinical hypothyroidism? Do we treat it in the same manner as overt hypothyroidism, or do we leave patients untreated since there are no symptoms?
Unfortunately, the answer is not cut and dry. A thyroid specialist can help to understand the symptoms, history, and nuance for each patient.
While subclinical hypothyroidism may associate with adverse cardiovascular, neuromuscular, metabolic, and cognitive effects, thyroid hormone replacement therapy has had mixed results. Studies on thyroid hormone replacement therapy for subclinical hypothyroidism do not conclude a definite answer on if the condition should be treated or not. Management should be individualized on a case by case basis.
With only slightly elevated TSH levels, thyroid dysfunction is unlikely to cause adverse outcomes. Though for some patients, treatment may be helpful. A thyroid specialist can help discuss the individual needs of patients in this range.
The likelihood that the condition progresses to overt hypothyroidism increases with higher TSH elevations and detectable antithyroid antibodies. Research does favor thyroid hormone replacement therapy in patients with persistent TSH elevation, progressively worsening TSH levels, elevated antithyroid antibodies, high lipid levels, cardiovascular disease or risk factors for cardiovascular disease, and anyone with typical hypothyroidism symptoms.
The management of subclinical hypothyroidism should base on the severity of the thyroid dysfunction, other health conditions, symptoms, and risk factors that may lead to a progression to overt hypothyroidism.
If you've been diagnosed with subclinical hypothyroidism, or are concerned that you have factors that put you at risk, have a conversation with your thyroid doctor.