{"id":4054,"date":"2019-09-26T17:31:50","date_gmt":"2019-09-26T17:31:50","guid":{"rendered":"http:\/\/ibshypo.com\/?page_id=4054"},"modified":"2021-12-27T11:07:58","modified_gmt":"2021-12-27T11:07:58","slug":"subnormal-tsh-secretion-my-case","status":"publish","type":"page","link":"https:\/\/ibshypo.com\/index.php\/subnormal-tsh-secretion-my-case\/","title":{"rendered":"Subnormal TSH – My Case"},"content":{"rendered":"

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There are several causes of subnormal TSH including starvation, severe illness or cardiac events. In these cases, the best approach is to try to resolve the underlying problem. Another cause of subnormal TSH is a down-regulated hypothalamic pituitary thyroid axis. This can occur after a period of TSH suppression for example from taking high dose thyroid hormone or from periods of thyrotoxicity in early-stage Hashimoto’s thyroiditis (which the patient may not notice) or in Graves’ disease.<\/p>\n\n\n\n

Long-term TSH suppression lowers the thyroid axis setpoint and fT3
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This study demonstrates that long-term TSH suppression down-regulates the hypothalamic pituitary thyroid axis. i.e. for given fT3 and fT4 levels TSH is lower than expected. In this study the dose of L-T4 needed to achieve TSH suppression dropped from 2.96 \u03bcg\/kg body weight to 2.25 \u03bcg\/kg body weight over a period of 12.7 years. Some of this reduction was independent of aging. The fT3 \/ fT4 ratio also dropped significantly<\/strong>.<\/p>\n\n\n\n

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Verburg, F & M\u00e4der, U & Grelle, I & Visser, Theo & Peeters, Robin & Smit, J & Reiners, Christoph. (2014). The Thyroid Axis ‘Setpoints’ are Significantly Altered After Long-Term Suppressive LT4 Therapy. Hormone and Metabolic Research. 46. 10.1055\/s-0034-1375678<\/a>. <\/strong><\/figcaption><\/figure><\/div>\n\n\n\n


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Results from my own trial<\/h3>\n\n\n\n

The following shows how a long-term suppressed TSH can down-regulate the axis and lead to reduced D2 activity causing hypothyroidism with ‘normal’ blood test results. It wasn’t intended as a prospective trial. After resolving my ARTH<\/a> I tried to come off thyroid tablets, my thyroid has always been healthy and there seemed no reason why I should continue taking hormone.<\/p>\n\n\n\n

T3 in the blood is thought to be 4x to 5x as potent as T4, at least in terms of its ability to suppress TSH. (Note this does not apply to T3 and T4 in tablet form, they have different absorption rates and half-lives). As a very rough guide I set up: –<\/p>\n\n\n\n

Set-Point Index =  (4 x fT3) + fT4 with a Mean of 36.8 calculated from the mid-points of the reference intervals.<\/p>\n\n\n\n

My % Mean from October 2000 (prior to any hormone treatment ) was 95%.<\/strong> At the time I had perfect blood test results TSH 1.0 (0.4 \u2013 5.5) fT3 4.9 (3.5 \u2013 6.5) fT4 13.3 (9.0 \u2013 20.0) <\/p>\n\n\n\n

The results below are from a period of about a year when I was on levothyroxine only.  When taking no medication, I was hypothyroid with tiredness, mild IBS and experienced considerable back pain when walking.  I was gradually getting worse – some consequences of hypothyroidism accumulate over time. Note that my Set-Point Index had fallen from 95% to 83%.<\/p>\n\n\n\n

On 100 \u03bcg L-T4 I felt almost euthyroid but still had substantial cognitive impairment.  The increase from 100\u03bcg to 125\u03bcg made no difference to my symptoms.<\/p>\n\n\n\n

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Patients 1 and 3 referred to earlier on in the Real Cases<\/a> section have blood tests with the same reference intervals making it easy to calculate their Set-Point Indexes: –<\/p>\n\n\n\n