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Subclinical Hypothyroidism

Subclinical hypothyroidism happens when you have elevated thyroid-stimulating hormone (TSH) levels with normal thyroxine levels. It usually doesn’t cause symptoms, and it may or may not require treatment.

Overview

What is subclinical hypothyroidism?

Subclinical hypothyroidism happens when you have elevated thyroid-stimulating hormone (TSH) levels with normal levels of thyroxine (T4). You don’t technically have hypothyroidism (commonly called overt hypothyroidism, in comparison), but it has the potential to develop into overt hypothyroidism.

Hypothyroidism happens when your thyroid doesn’t produce enough thyroid hormones (thyroxine and triiodothyronine). “Subclinical” describes a condition that’s not severe enough to cause definite symptoms.

Thyroid-stimulating hormone, commonly called TSH and also referred to as thyrotropin, is a hormone that your pituitary gland releases to trigger your thyroid to produce and release its own hormones — thyroxine (T4) and triiodothyronine (T3). These two hormones are essential for maintaining your body’s metabolism — how your body transforms the food you eat into energy and uses it.

Subclinical hypothyroidism is often temporary but can be long-lasting. It may or may not require treatment.

Who does subclinical hypothyroidism affect?

Anyone can have subclinical hypothyroidism, but it’s more likely to affect adults assigned female at birth and people over the age of 65.

Does subclinical hypothyroidism affect pregnancy?

Subclinical hypothyroidism is more common during pregnancy than overt hypothyroidism. It affects 15% to 28% of pregnant people.

Evidence linking subclinical hypothyroidism to issues during pregnancy is inconsistent and conflicting. Older studies have shown an association between subclinical hypothyroidism in pregnancy and the following conditions:

Hypertensive disorders of pregnancy, including gestational high blood pressure and preeclampsia.
Premature (preterm) labor.
Impaired cognitive development in infants.

However, more recent studies have not replicated these associations.

Healthcare providers typically only screen for subclinical hypothyroidism during pregnancy if you have risk factors for developing it, including:

Personal or family history of thyroid disease.
Positive thyroid antibodies.
Type 1 diabetes and other autoimmune diseases.
History of preterm delivery, miscarriage and/or infertility.
Two or more previous pregnancies.
Prior or current amiodarone or lithium use.
Head or neck radiation exposure.
Class III obesity.
Age older than 30 years.

Pregnant people who have subclinical hypothyroidism and thyroid peroxidase (TPO) antibodies require thyroid replacement therapy (levothyroxine). Most people with subclinical hypothyroidism in pregnancy won’t require treatment postpartum (after pregnancy).

How common is subclinical hypothyroidism?

Subclinical hypothyroidism is common. It affects up to 10% of adults in the United States.

Symptoms and Causes

What are the symptoms of subclinical hypothyroidism?

Most of the time, subclinical hypothyroidism doesn’t cause any symptoms (it’s asymptomatic).

However, it can sometimes present with mild symptoms of hypothyroidism, which include:

Fatigue.
Unexplained weight gain.
Constipation.
Depression and/or decreased attention span.
Being unable to tolerate cold temperatures.
Dry, coarse skin and hair.
Diastolic hypertension (high blood pressure).
Frequent and heavy menstrual bleeding.

What causes subclinical hypothyroidism?

Normally, multiple hormones and glands in your endocrine system work together to carefully control the level of TSH in your bloodstream through a feedback loop.

To start, your hypothalamus releases thyroid-releasing hormone (TRH) to trigger the release of thyroid-stimulating hormone (TSH) by your pituitary gland.

TSH then stimulates cells in your thyroid to release thyroxine or T4 (80%) and triiodothyronine or T3 (20%) into your bloodstream. These two hormones prevent your pituitary gland from producing more TSH if the levels of thyroxine and triiodothyronine are too high, thus completing the cycle. When T4 and T3 levels drop, the cycle starts over again.

However, in subclinical hypothyroidism, due to thyroid inflammation or other thyroid disease, thyroid hormonal output doesn’t increase like it normally should in response to the elevated TSH levels. This leads to elevated TSH levels and normal thyroxine (T4) levels, resulting in subclinical hypothyroidism.

Diagnosis and Tests

How is subclinical hypothyroidism diagnosed?

The diagnosis of subclinical hypothyroidism is solely based on thyroid function testing (thyroid blood tests).

The normal test range for thyroid-stimulating hormone (TSH or thyrotropin) for a non-pregnant adult is 0.4 to 4.5 mIU/L (milli-international units per liter of blood). The normal range for TSH levels for pregnant people varies by trimester.

If you had thyroid blood tests and the results indicate that your TSH levels are elevated (5 to 10 mIU/L) and your thyroxine (T4) levels are in the normal range, it means you have subclinical hypothyroidism.

Subclinical hypothyroidism may be categorized as grade 1 when TSH levels are 4.5 and 9.9 mIU/L and as grade 2 if TSH levels are 10 mIU/L or higher. Approximately 90% of people with subclinical hypothyroidism have TSH levels lower than 10 mIU/L.

Management and Treatment

Does subclinical hypothyroidism need to be treated?

Healthcare providers disagree on whether subclinical hypothyroidism needs to be treated due to conflicting studies showing its effectiveness.

In theory, the reasoning for treating subclinical hypothyroidism would be to decrease the risk of cardiovascular issues and potentially prevent it from progressing to overt hypothyroidism.

However, the reason for not treating subclinical hypothyroidism is that treatment could potentially cause thyrotoxicosis (too much thyroid hormone in your body), especially in people aged 65 years or older. In addition, most people with subclinical hypothyroidism don’t have symptoms.

For most people with subclinical hypothyroidism, providers recommend that they take a “wait and see” approach and not start treatment to see if the subclinical hypothyroidism resolves on its own. However, providers may recommend treatment in the following cases:

People who have thyrotropin (TSH) levels of 10 mIU/L or higher.
Young and middle-aged people who have symptoms of mild hypothyroidism.
Young and middle-aged people who have other cardiovascular disease risk factors.

Subclinical hypothyroidism treatment for fertility

The American Thyroid Association recommends that people assigned female at birth with subclinical hypothyroidism who are having in vitro fertilization (IVF) or intracytoplasmic sperm injection (ICSI) be treated with levothyroxine to reach a TSH level of 2.5 mIU/L.

What is the treatment for subclinical hypothyroidism?

If your healthcare provider recommends treatment for subclinical hypothyroidism, they’ll prescribe a thyroid hormone replacement medication called levothyroxine. It comes in pill form.

Before starting levothyroxine therapy in subclinical hypothyroidism, your provider may order another blood test to check your TSH levels within three months of the first abnormal test result. This is because the TSH level normalizes in about 60% of cases after three months.

Your provider will likely want you to get routine blood tests to make sure your thyroid levels are in a healthy range while taking medication. If your dosage of levothyroxine is too high, it can cause hyperthyroidism.

Can you get rid of subclinical hypothyroidism?

While there’s nothing you can do yourself to get rid of subclinical hypothyroidism, it often — but not always — goes away on its own with time.

Otherwise, medication can treat subclinical hypothyroidism, but healthcare providers don’t always recommend treatment.

Newborns with borderline thyroid function at higher risk of poor neurodevelopmental outcomes

Date: July 28, 2016 Source: University of Sydney Summary: Babies born with moderately high concentrations of thyroid stimulating hormone have a higher risk of poor educational and development outcomes at school age, a world-first study reveals. Share:

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FULL STORY

Babies born with moderately high concentrations of thyroid stimulating hormone have a higher risk of poor educational and development outcomes at school age, a world-first University of Sydney study reveals.

Published in the latest issue of The Lancet Diabetes & Endocrinology, this is the first population-based study demonstrating the association between moderately high thyroid stimulating hormone (TSH) concentrations in infants and their later school age neurodevelopmental outcomes.

Congenital hypothyroidism refers to abnormal thyroid function in newborn infants. Globally, about one in 2,000 children are born with congenital hypothyroidism each year and the incidence of subclinical thyroid disease is at least ten times higher than overt thyroid disease. If untreated for several months after birth, severe congenital hypothyroidism can lead to growth failure and permanent intellectual disability.

Screening for congenital hypothyroidism in the first days of life, done usually by testing concentrations of neonatal thyroid-stimulating hormone (TSH) in baby’s blood, provides an opportunity to identify infants with abnormal thyroid hormone concentrations.

In developed countries, newborn screening of TSH levels and early treatment for congenital hypothyroidism has nearly eliminated intellectual disabilities associated with congenital hypothyroidism. Currently, only newborns with TSH concentrations at the 99.95th percentile of the population range are diagnosed with congenital hypothyroidism and treated with thyroxine. At this percentile, blood concentration of TSH usually exceeds 20 mU per litre of whole blood.

The researchers found that infants with a neonatal TSH concentration lower than the newborn screening cut-off (20 mU/L blood) but in the top quartile of the population have an increased likelihood of poor neurodevelopmental outcomes at school age. Said another way, the study reveals a gradual increasing risk of poor educational and development outcomes for newborns with increasing TSH concentrations from the 75th to the 99.95th percentile.

Other findings

Compared to those with normal TSH levels (less than 75th percentile TSH concentrations) children had a 42 per cent higher risk of poor reading performance when TSH concentrations ranged between the 99.9th to 99.95th percentiles.

Compared to those with normal TSH levels (less than 75th percentile TSH concentrations) children had a 52 per cent higher risk of vulnerability in developmental domains when TSH concentrations ranged between the 99.9th to 99.5th percentiles.

Compared to those with normal TSH levels (less than 75th percentile TSH concentrations) children had a 68 per cent higher risk of having ‘special needs’ when TSH concentrations ranged between the 99.9th to 99.5th percentiles.

«The results showed a clear dose-response association between neonatal thyroid stimulating hormone and risk of scoring below the national minimum standard for numeracy and reading,» said the University of Sydney’s A/Professor Natasha Nassar, the study’s senior author.

«This study can’t prove a cause and effect relationship between thyroid stimulating hormone levels in newborns and educational and development outcomes in school age children, but it suggests an urgent need for prospective studies examining different thyroid hormone thresholds for intervening with thyroxine,» said Dr Bridget Wilcken, Clinical Professor of Paediatrics and Child Health at the Children’s Hospital at Westmead.

«Given that thyroxine is a relatively safe medication when indicated and properly monitored, this simple intervention may prevent significant learning and developmental problems in a small group of affected children.

A/Professor Nassar said: «The study also demonstrates the power of data linkage to provide a cost-effective platform to answer important research questions at a population and individual level.»

RELATED TOPICS
- Health & Medicine
  - Thyroid Disease
  - Hormone Disorders
  - Infant’s Health
  - Children’s Health
  - Child Psychology
  - Learning Disorders
  - Child Development
  - Infant and Preschool Learning
  - RELATED TERMS
    - Pituitary gland
    - Premature birth
    - MMR vaccine
    - Thyroid
    - Hypothyroidism
    - Thyroid hormone
    - Education
    - Pregnancy
    Mayo Clinic Q and A: Several factors to consider before treating hypothyroidism
    
    DEAR MAYO CLINIC: At my last checkup, my doctor told me I have borderline hypothyroidism and gave me a prescription for medication to treat it. She said she would check my thyroid again in six months. Is this something I will have to take for the rest of my life? What are the risks if I choose not to take the medicine? I am a 62-year-old woman and very healthy.
    ANSWER: Before you move forward with treatment for hypothyroidism, it would be worthwhile to wait and repeat the test in several months to confirm your diagnosis. Even if the results are the same at that time, you should consider several other factors before you decide on treatment.
    Your thyroid is a small, butterfly-shaped gland at the base of the front of your neck. Hypothyroidism, sometimes called underactive thyroid, is a condition in which your thyroid gland doesn’t produce enough of certain important hormones. The hormones that the thyroid gland makes — triiodothyronine, or T3, and thyroxine, or T4 — have a large impact on your health, affecting all aspects of your metabolism. They maintain the rate at which your body uses fats and carbohydrates, help control your body temperature, influence your heart rate, and help regulate the production of proteins.
    The rate at which your thyroid makes T3 and T4 is regulated by another hormone that your pituitary gland produces, called thyroid-stimulating hormone, or TSH. The term “borderline hypothyroidism” typically is used when blood tests show that your body’s level of TSH is slightly above normal, but your T3 and T4 levels are normal. Another name for this condition is subclinical hypothyroidism.
    Not all physicians agree on whether there is benefit to treating hypothyroidism at this stage. That’s because treatment typically involves a daily dose of a synthetic hormone, and if you take that medication in excessive doses it can have a negative effect on a variety of your body’s systems, including your brain, heart and muscle function. It also can interfere with how your body handles fluid and fats.
    If left untreated, about 30 percent of people whose condition falls into the category of subclinical hypothyroidism have their TSH levels return to normal within one year. Only 3 percent per year will go on to develop the classical form of hypothyroidism. That condition is characterized by elevated TSH levels and low levels of T3 and T4. It requires treatment in all cases.
    If a second blood test confirms your diagnosis of subclinical hypothyroidism, there are several factors to consider before you decide on treatment. In general, treatment is recommended if you have symptoms of hypothyroidism, such as fluid retention, fatigue, increased sensitivity to cold, constipation, muscle weakness or painful joints, among others. Treatment also may be necessary if you have another underlying medical condition, such as congestive heart failure or high cholesterol.
    In these cases of subclinical hypothyroidism, many physicians recommend treatment for three to six months to see if it helps relieve symptoms. If after the initial course of treatment the symptoms remain, then treatment needs to be re-evaluated.
    If you have subclinical hypothyroidism, but you don’t have symptoms or other health problems, then it would be wise to wait with treatment and be tested again in six to 12 months. If your TSH level is significantly higher; if it increases consistently and you have a family history of thyroid disease; or if another blood test finds you have positive anti-thyroid antibodies, therapy would be appropriate at that time. — Marius N. Stan, M.D., Endocrinology, Mayo Clinic, Rochester, Minn.