Summary

Dr. Alaka Deshpande covers..

• conditions to be ruled out before diagnosing subclinical hypothyroidism (SCH)
• management of SCH; debate on thyroxine therapy

The easy availability of hormone assays is often misused in clinical practice particularly for thyroid hormones. It has been observed that apart from modern medicine practitioners, the thyroid hormone studies are advised by practitioners of AYUSH, nutritionist and gym trainers particularly for overweight people. The interpretation of results is a challenge.

The commonest cause of primary hypothyroidism is an autoimmune thyroid disorder (AITD). But most of the times the thyroid antibodies are not measured.

Primary Hypothyroidism is diagnosed when serum FT3, FT4 are low and serum TSH is elevated to more than 10 µIU/ml. Serum thyroperoxidase (TPO) antibody levels are high.

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What is subclinical hypothyroidism (SCH)?

It is defined as serum TSH (S.TSH) levels are above the upper limit of normal but less than 10 µIU/ml despite NORMAL S.FT4.

There is a log-linear relationship between circulating thyroid hormone level and S.TSH. A two-fold change in FT4 will produce a 100-fold change in TSH.

Therefore S.TSH estimation is necessary for diagnosis of early or mild thyroid failure when peripheral hormone levels are within normal laboratory range.

Before diagnosing SCH the following conditions need to be ruled out:

• Assay variability
• Recovery from non—thyroidal illness
• Previous radioiodine therapy
• External radiation therapy
• Thyroid surgery
• Transient post-partum thyroiditis
• Granulomatous thyroiditis

National Health and Nutition survey III has suggested that S.TSH distribution progressively shifts to higher concentration with advancing age thus overestimating SCH in advancing age, age specific range in the community needs to be studied.

SCH is a common problem with a prevalence of 3 to 8% in the population without known thyroid disease. It increases with age and higher in women.

TPO antibodies can be detected in 80% of SCH patients and 80% of SCH cases have S.TSH less than 10 µIU/ml.

2.6% of SCH cases with absent thyroid antibodies progress to overt hypothyroidism each year as against 4.3% SCH progressing to overt hypothyroidism with presence of antibodies.

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Management of SCH

Does a patient of SCH with normal level of thyroid hormones with S.TSH varying between 5 -10 µIU/ml benefit by thyroxine therapy ?

There is no consensus regarding levothyroxine therapy. American thyroid guidelines state S.TSH >10 is an overt hypothyroidism and should be treated with levothyroxine.

In case of SCH it is advised:

1. To measure S.TPO Abs
2. Follow up patient with S.TSH every 6 months
3. As and when patient develops overt hypothyroidism ie S.TSH >10 the levothyroxine can be started

However clinicians can use their discretion in SCH cases with High TPO Abs levels if:

• H/O thyroid dysfunction is present in family members
• The patient has infertility/bad obstetric history
• The patient has menstrual irregularity
• It is a case of unexpected Pregnancy
• In case of hypercholesterolemia

The benefit of levothyroxine treatment in all the above conditions lacks an unequivocal evidence.

If at all the treatment is to be given it should start with smaller dose of 25 to 50 mcg per day with regular monitoring of S.TSH levels.

In my experience, I have seen a large number of cases of SCH being started without indication on Thyroxine 100 mcg/day. Such inadvertent use may result in iatrogenic thyrotoxicosis or may lead to osteoporosis.

As per present guidelines, it is better to reassure the patient and estimate S.TSH every 6 months.

Thyroxine is NOT the treatment for weight loss or obesity which needs to be treated with low calorie diet and motivation to sustain it.

Disclaimer- The views and opinions expressed in this article are those of the author's and do not necessarily reflect the official policy or position of M3 India.

This article was originally published on 27.03.19.